Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being “Free Software” and “Free Software Needs
Free Documentation”, with the Front-Cover Texts being “A GNU Manual,”
and with the Back-Cover Texts as in (a) below.

Support for Modula-2 is partial. For information on Modula-2, see
Modula-2.

Support for OpenCL C is partial. For information on OpenCL C, see
OpenCL C.

Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work. GDB does not support
entering expressions, printing values, or similar features using Pascal
syntax.

GDB can be used to debug programs written in Fortran, although
it may be necessary to refer to some variables with a trailing
underscore.

GDB can be used to debug programs written in Objective-C,
using either the Apple/NeXT or the GNU Objective-C runtime.

Free Software

GDB is free software, protected by the GNU
General Public License
(GPL). The GPL gives you the freedom to copy or adapt a licensed
program—but every person getting a copy also gets with it the
freedom to modify that copy (which means that they must get access to
the source code), and the freedom to distribute further copies.
Typical software companies use copyrights to limit your freedoms; the
Free Software Foundation uses the GPL to preserve these freedoms.

Fundamentally, the General Public License is a license which says that
you have these freedoms and that you cannot take these freedoms away
from anyone else.

Free Software Needs Free Documentation

The biggest deficiency in the free software community today is not in
the software—it is the lack of good free documentation that we can
include with the free software. Many of our most important
programs do not come with free reference manuals and free introductory
texts. Documentation is an essential part of any software package;
when an important free software package does not come with a free
manual and a free tutorial, that is a major gap. We have many such
gaps today.

Consider Perl, for instance. The tutorial manuals that people
normally use are non-free. How did this come about? Because the
authors of those manuals published them with restrictive terms—no
copying, no modification, source files not available—which exclude
them from the free software world.

That wasn’t the first time this sort of thing happened, and it was far
from the last. Many times we have heard a GNU user eagerly describe a
manual that he is writing, his intended contribution to the community,
only to learn that he had ruined everything by signing a publication
contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not
price. The problem with the non-free manual is not that publishers
charge a price for printed copies—that in itself is fine. (The Free
Software Foundation sells printed copies of manuals, too.) The
problem is the restrictions on the use of the manual. Free manuals
are available in source code form, and give you permission to copy and
modify. Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for
free software. Redistribution (including the normal kinds of
commercial redistribution) must be permitted, so that the manual can
accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too.
When people modify the software, adding or changing features, if they
are conscientious they will change the manual too—so they can
provide accurate and clear documentation for the modified program. A
manual that leaves you no choice but to write a new manual to document
a changed version of the program is not really available to our
community.

Some kinds of limits on the way modification is handled are
acceptable. For example, requirements to preserve the original
author’s copyright notice, the distribution terms, or the list of
authors, are ok. It is also no problem to require modified versions
to include notice that they were modified. Even entire sections that
may not be deleted or changed are acceptable, as long as they deal
with nontechnical topics (like this one). These kinds of restrictions
are acceptable because they don’t obstruct the community’s normal use
of the manual.

However, it must be possible to modify all the technical
content of the manual, and then distribute the result in all the usual
media, through all the usual channels. Otherwise, the restrictions
obstruct the use of the manual, it is not free, and we need another
manual to replace it.

Please spread the word about this issue. Our community continues to
lose manuals to proprietary publishing. If we spread the word that
free software needs free reference manuals and free tutorials, perhaps
the next person who wants to contribute by writing documentation will
realize, before it is too late, that only free manuals contribute to
the free software community.

If you are writing documentation, please insist on publishing it under
the GNU Free Documentation License or another free documentation
license. Remember that this decision requires your approval—you
don’t have to let the publisher decide. Some commercial publishers
will use a free license if you insist, but they will not propose the
option; it is up to you to raise the issue and say firmly that this is
what you want. If the publisher you are dealing with refuses, please
try other publishers. If you’re not sure whether a proposed license
is free, write to licensing@gnu.org.

You can encourage commercial publishers to sell more free, copylefted
manuals and tutorials by buying them, and particularly by buying
copies from the publishers that paid for their writing or for major
improvements. Meanwhile, try to avoid buying non-free documentation
at all. Check the distribution terms of a manual before you buy it,
and insist that whoever seeks your business must respect your freedom.
Check the history of the book, and try to reward the publishers that
have paid or pay the authors to work on it.

Contributors to GDB

Richard Stallman was the original author of GDB, and of many
other GNU programs. Many others have contributed to its
development. This section attempts to credit major contributors. One
of the virtues of free software is that everyone is free to contribute
to it; with regret, we cannot actually acknowledge everyone here. The
file ChangeLog in the GDB distribution approximates a
blow-by-blow account.

Changes much prior to version 2.0 are lost in the mists of time.

Plea: Additions to this section are particularly welcome. If you
or your friends (or enemies, to be evenhanded) have been unfairly
omitted from this list, we would like to add your names!

Richard Stallman, assisted at various times by Peter TerMaat, Chris
Hanson, and Richard Mlynarik, handled releases through 2.8.

Michael Tiemann is the author of most of the GNU C++ support
in GDB, with significant additional contributions from Per
Bothner and Daniel Berlin. James Clark wrote the GNU C++
demangler. Early work on C++ was by Peter TerMaat (who also did
much general update work leading to release 3.0).

Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from
Tensilica, Inc. contributed support for Xtensa processors. Others
who have worked on the Xtensa port of GDB in the past include
Steve Tjiang, John Newlin, and Scott Foehner.

Michael Eager and staff of Xilinx, Inc., contributed support for the
Xilinx MicroBlaze architecture.

Initial support for the FreeBSD/mips target and native configuration
was developed by SRI International and the University of Cambridge
Computer Laboratory under DARPA/AFRL contract FA8750-10-C-0237
("CTSRD"), as part of the DARPA CRASH research programme.

The original port to the OpenRISC 1000 is believed to be due to
Alessandro Forin and Per Bothner. More recent ports have been the work
of Jeremy Bennett, Franck Jullien, Stefan Wallentowitz and
Stafford Horne.

1 A Sample GDB Session

You can use this manual at your leisure to read all about GDB.
However, a handful of commands are enough to get started using the
debugger. This chapter illustrates those commands.

One of the preliminary versions of GNUm4 (a generic macro
processor) exhibits the following bug: sometimes, when we change its
quote strings from the default, the commands used to capture one macro
definition within another stop working. In the following short m4
session, we define a macro foo which expands to 0000; we
then use the m4 built-in defn to define bar as the
same thing. However, when we change the open quote string to
<QUOTE> and the close quote string to <UNQUOTE>, the same
procedure fails to define a new synonym baz:

$ gdb m4
GDB is free software and you are welcome to distribute copies
of it under certain conditions; type "show copying" to see
the conditions.
There is absolutely no warranty for GDB; type "show warranty"
for details.
GDB 8.2.1.20190121-git, Copyright 1999 Free Software Foundation, Inc...
(gdb)

GDB reads only enough symbol data to know where to find the
rest when needed; as a result, the first prompt comes up very quickly.
We now tell GDB to use a narrower display width than usual, so
that examples fit in this manual.

(gdb) set width 70

We need to see how the m4 built-in changequote works.
Having looked at the source, we know the relevant subroutine is
m4_changequote, so we set a breakpoint there with the GDBbreak command.

The display that shows the subroutine where m4 is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt), to see where we are
in the stack as a whole: the backtrace command displays a
stack frame for each active subroutine.

The last line displayed looks a little odd; we can examine the variables
lquote and rquote to see if they are in fact the new left
and right quotes we specified. We use the command p
(print) to see their values.

That certainly looks wrong, assuming len_lquote and
len_rquote are meant to be the lengths of lquote and
rquote respectively. We can set them to better values using
the p command, since it can print the value of
any expression—and that expression can include subroutine calls and
assignments.

Is that enough to fix the problem of using the new quotes with the
m4 built-in defn? We can allow m4 to continue
executing with the c (continue) command, and then try the
example that caused trouble initially:

(gdb) c
Continuing.
define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
0000

Success! The new quotes now work just as well as the default ones. The
problem seems to have been just the two typos defining the wrong
lengths. We allow m4 exit by giving it an EOF as input:

Ctrl-d
Program exited normally.

The message ‘Program exited normally.’ is from GDB; it
indicates m4 has finished executing. We can end our GDB
session with the GDBquit command.

2.1 Invoking GDB

Invoke GDB by running the program gdb. Once started,
GDB reads commands from the terminal until you tell it to exit.

You can also run gdb with a variety of arguments and options,
to specify more of your debugging environment at the outset.

The command-line options described here are designed
to cover a variety of situations; in some environments, some of these
options may effectively be unavailable.

The most usual way to start GDB is with one argument,
specifying an executable program:

gdb program

You can also start with both an executable program and a core file
specified:

gdb programcore

You can, instead, specify a process ID as a second argument, if you want
to debug a running process:

gdb program 1234

would attach GDB to process 1234 (unless you also have a file
named 1234; GDB does check for a core file first).

Taking advantage of the second command-line argument requires a fairly
complete operating system; when you use GDB as a remote
debugger attached to a bare board, there may not be any notion of
“process”, and there is often no way to get a core dump. GDB
will warn you if it is unable to attach or to read core dumps.

You can optionally have gdb pass any arguments after the
executable file to the inferior using --args. This option stops
option processing.

gdb --args gcc -O2 -c foo.c

This will cause gdb to debug gcc, and to set
gcc’s command-line arguments (see Arguments) to ‘-O2 -c foo.c’.

You can run gdb without printing the front material, which describes
GDB’s non-warranty, by specifying --silent
(or -q/--quiet):

gdb --silent

You can further control how GDB starts up by using command-line
options. GDB itself can remind you of the options available.

Type

gdb -help

to display all available options and briefly describe their use
(‘gdb -h’ is a shorter equivalent).

All options and command line arguments you give are processed
in sequential order. The order makes a difference when the
‘-x’ option is used.

2.1.1 Choosing Files

When GDB starts, it reads any arguments other than options as
specifying an executable file and core file (or process ID). This is
the same as if the arguments were specified by the ‘-se’ and
‘-c’ (or ‘-p’) options respectively. (GDB reads the
first argument that does not have an associated option flag as
equivalent to the ‘-se’ option followed by that argument; and the
second argument that does not have an associated option flag, if any, as
equivalent to the ‘-c’/‘-p’ option followed by that argument.)
If the second argument begins with a decimal digit, GDB will
first attempt to attach to it as a process, and if that fails, attempt
to open it as a corefile. If you have a corefile whose name begins with
a digit, you can prevent GDB from treating it as a pid by
prefixing it with ./, e.g. ./12345.

If GDB has not been configured to included core file support,
such as for most embedded targets, then it will complain about a second
argument and ignore it.

Many options have both long and short forms; both are shown in the
following list. GDB also recognizes the long forms if you truncate
them, so long as enough of the option is present to be unambiguous.
(If you prefer, you can flag option arguments with ‘--’ rather
than ‘-’, though we illustrate the more usual convention.)

-symbols file

-s file

Read symbol table from file file.

-exec file

-e file

Use file file as the executable file to execute when appropriate,
and for examining pure data in conjunction with a core dump.

-se file

Read symbol table from file file and use it as the executable
file.

-core file

-c file

Use file file as a core dump to examine.

-pid number

-p number

Connect to process ID number, as with the attach command.

-command file

-x file

Execute commands from file file. The contents of this file is
evaluated exactly as the source command would.
See Command files.

-eval-command command

-ex command

Execute a single GDB command.

This option may be used multiple times to call multiple commands. It may
also be interleaved with ‘-command’ as required.

gdb -ex 'target sim' -ex 'load' \
-x setbreakpoints -ex 'run' a.out

-init-command file

-ix file

Execute commands from file file before loading the inferior (but
after loading gdbinit files).
See Startup.

-init-eval-command command

-iex command

Execute a single GDB command before loading the inferior (but
after loading gdbinit files).
See Startup.

-directory directory

-d directory

Add directory to the path to search for source and script files.

-r

-readnow

Read each symbol file’s entire symbol table immediately, rather than
the default, which is to read it incrementally as it is needed.
This makes startup slower, but makes future operations faster.

--readnever

Do not read each symbol file’s symbolic debug information. This makes
startup faster but at the expense of not being able to perform
symbolic debugging. DWARF unwind information is also not read,
meaning backtraces may become incomplete or inaccurate. One use of
this is when a user simply wants to do the following sequence: attach,
dump core, detach. Loading the debugging information in this case is
an unnecessary cause of delay.

2.1.2 Choosing Modes

You can run GDB in various alternative modes—for example, in
batch mode or quiet mode.

-nx

-n

Do not execute commands found in any initialization file.
There are three init files, loaded in the following order:

system.gdbinit

This is the system-wide init file.
Its location is specified with the --with-system-gdbinit
configure option (see System-wide configuration).
It is loaded first when GDB starts, before command line options
have been processed.

~/.gdbinit

This is the init file in your home directory.
It is loaded next, after system.gdbinit, and before
command options have been processed.

./.gdbinit

This is the init file in the current directory.
It is loaded last, after command line options other than -x and
-ex have been processed. Command line options -x and
-ex are processed last, after ./.gdbinit has been loaded.

For further documentation on startup processing, See Startup.
For documentation on how to write command files,
See Command Files.

-nh

Do not execute commands found in ~/.gdbinit, the init file
in your home directory.
See Startup.

-quiet

-silent

-q

“Quiet”. Do not print the introductory and copyright messages. These
messages are also suppressed in batch mode.

-batch

Run in batch mode. Exit with status 0 after processing all the
command files specified with ‘-x’ (and all commands from
initialization files, if not inhibited with ‘-n’). Exit with
nonzero status if an error occurs in executing the GDB commands
in the command files. Batch mode also disables pagination, sets unlimited
terminal width and height see Screen Size, and acts as if set confirm
off were in effect (see Messages/Warnings).

Batch mode may be useful for running GDB as a filter, for
example to download and run a program on another computer; in order to
make this more useful, the message

Program exited normally.

(which is ordinarily issued whenever a program running under
GDB control terminates) is not issued when running in batch
mode.

-batch-silent

Run in batch mode exactly like ‘-batch’, but totally silently. All
GDB output to stdout is prevented (stderr is
unaffected). This is much quieter than ‘-silent’ and would be useless
for an interactive session.

This is particularly useful when using targets that give ‘Loading section’
messages, for example.

Note that targets that give their output via GDB, as opposed to
writing directly to stdout, will also be made silent.

-return-child-result

The return code from GDB will be the return code from the child
process (the process being debugged), with the following exceptions:

GDB exits abnormally. E.g., due to an incorrect argument or an
internal error. In this case the exit code is the same as it would have been
without ‘-return-child-result’.

The user quits with an explicit value. E.g., ‘quit 1’.

The child process never runs, or is not allowed to terminate, in which case
the exit code will be -1.

This option is useful in conjunction with ‘-batch’ or ‘-batch-silent’,
when GDB is being used as a remote program loader or simulator
interface.

-nowindows

-nw

“No windows”. If GDB comes with a graphical user interface
(GUI) built in, then this option tells GDB to only use the command-line
interface. If no GUI is available, this option has no effect.

-windows

-w

If GDB includes a GUI, then this option requires it to be
used if possible.

-cd directory

Run GDB using directory as its working directory,
instead of the current directory.

-data-directory directory

-D directory

Run GDB using directory as its data directory.
The data directory is where GDB searches for its
auxiliary files. See Data Files.

-fullname

-f

GNU Emacs sets this option when it runs GDB as a
subprocess. It tells GDB to output the full file name and line
number in a standard, recognizable fashion each time a stack frame is
displayed (which includes each time your program stops). This
recognizable format looks like two ‘\032’ characters, followed by
the file name, line number and character position separated by colons,
and a newline. The Emacs-to-GDB interface program uses the two
‘\032’ characters as a signal to display the source code for the
frame.

-annotate level

This option sets the annotation level inside GDB. Its
effect is identical to using ‘set annotate level’
(see Annotations). The annotation level controls how much
information GDB prints together with its prompt, values of
expressions, source lines, and other types of output. Level 0 is the
normal, level 1 is for use when GDB is run as a subprocess of
GNU Emacs, level 3 is the maximum annotation suitable for programs
that control GDB, and level 2 has been deprecated.

The annotation mechanism has largely been superseded by GDB/MI
(see GDB/MI).

--args

Change interpretation of command line so that arguments following the
executable file are passed as command line arguments to the inferior.
This option stops option processing.

-baud bps

-b bps

Set the line speed (baud rate or bits per second) of any serial
interface used by GDB for remote debugging.

-l timeout

Set the timeout (in seconds) of any communication used by GDB
for remote debugging.

-tty device

-t device

Run using device for your program’s standard input and output.

-tui

Activate the Text User Interface when starting. The Text User
Interface manages several text windows on the terminal, showing
source, assembly, registers and GDB command outputs
(see GDB Text User Interface). Do not use this
option if you run GDB from Emacs (see Using GDB under GNU Emacs).

-interpreter interp

Use the interpreter interp for interface with the controlling
program or device. This option is meant to be set by programs which
communicate with GDB using it as a back end.
See Command Interpreters.

‘--interpreter=mi’ (or ‘--interpreter=mi2’) causes
GDB to use the GDB/MI interface (see The GDB/MI Interface) included since GDB version 6.0. The
previous GDB/MI interface, included in GDB version 5.3 and
selected with ‘--interpreter=mi1’, is deprecated. Earlier
GDB/MI interfaces are no longer supported.

-write

Open the executable and core files for both reading and writing. This
is equivalent to the ‘set write on’ command inside GDB
(see Patching).

-statistics

This option causes GDB to print statistics about time and
memory usage after it completes each command and returns to the prompt.

-version

This option causes GDB to print its version number and
no-warranty blurb, and exit.

-configuration

This option causes GDB to print details about its build-time
configuration parameters, and then exit. These details can be
important when reporting GDB bugs (see GDB Bugs).

Reads the init file (if any) in your home directory1 and executes all the commands in
that file.

Executes commands and command files specified by the ‘-iex’ and
‘-ix’ options in their specified order. Usually you should use the
‘-ex’ and ‘-x’ options instead, but this way you can apply
settings before GDB init files get executed and before inferior
gets loaded.

Processes command line options and operands.

Reads and executes the commands from init file (if any) in the current
working directory as long as ‘set auto-load local-gdbinit’ is set to
‘on’ (see Init File in the Current Directory).
This is only done if the current directory is
different from your home directory. Thus, you can have more than one
init file, one generic in your home directory, and another, specific
to the program you are debugging, in the directory where you invoke
GDB.

If the command line specified a program to debug, or a process to
attach to, or a core file, GDB loads any auto-loaded
scripts provided for the program or for its loaded shared libraries.
See Auto-loading.

If you wish to disable the auto-loading during startup,
you must do something like the following:

$ gdb -iex "set auto-load python-scripts off" myprogram

Option ‘-ex’ does not work because the auto-loading is then turned
off too late.

Executes commands and command files specified by the ‘-ex’ and
‘-x’ options in their specified order. See Command Files, for
more details about GDB command files.

Reads the command history recorded in the history file.
See Command History, for more details about the command history and the
files where GDB records it.

Init files use the same syntax as command files (see Command Files) and are processed by GDB in the same way. The init
file in your home directory can set options (such as ‘set
complaints’) that affect subsequent processing of command line options
and operands. Init files are not executed if you use the ‘-nx’
option (see Choosing Modes).

To display the list of init files loaded by gdb at startup, you
can use gdb --help.

The GDB init files are normally called .gdbinit.
The DJGPP port of GDB uses the name gdb.ini, due to
the limitations of file names imposed by DOS filesystems. The Windows
port of GDB uses the standard name, but if it finds a
gdb.ini file in your home directory, it warns you about that
and suggests to rename the file to the standard name.

2.2 Quitting GDB

quit [expression]

q

To exit GDB, use the quit command (abbreviated
q), or type an end-of-file character (usually Ctrl-d). If you
do not supply expression, GDB will terminate normally;
otherwise it will terminate using the result of expression as the
error code.

An interrupt (often Ctrl-c) does not exit from GDB, but rather
terminates the action of any GDB command that is in progress and
returns to GDB command level. It is safe to type the interrupt
character at any time because GDB does not allow it to take effect
until a time when it is safe.

2.3 Shell Commands

If you need to execute occasional shell commands during your
debugging session, there is no need to leave or suspend GDB; you can
just use the shell command.

shell command-string

!command-string

Invoke a standard shell to execute command-string.
Note that no space is needed between ! and command-string.
If it exists, the environment variable SHELL determines which
shell to run. Otherwise GDB uses the default shell
(/bin/sh on Unix systems, COMMAND.COM on MS-DOS, etc.).

The utility make is often needed in development environments.
You do not have to use the shell command for this purpose in
GDB:

make make-args

Execute the make program with the specified
arguments. This is equivalent to ‘shell make make-args’.

3 GDB Commands

You can abbreviate a GDB command to the first few letters of the command
name, if that abbreviation is unambiguous; and you can repeat certain
GDB commands by typing just RET. You can also use the TAB
key to get GDB to fill out the rest of a word in a command (or to
show you the alternatives available, if there is more than one possibility).

3.1 Command Syntax

A GDB command is a single line of input. There is no limit on
how long it can be. It starts with a command name, which is followed by
arguments whose meaning depends on the command name. For example, the
command step accepts an argument which is the number of times to
step, as in ‘step 5’. You can also use the step command
with no arguments. Some commands do not allow any arguments.

GDB command names may always be truncated if that abbreviation is
unambiguous. Other possible command abbreviations are listed in the
documentation for individual commands. In some cases, even ambiguous
abbreviations are allowed; for example, s is specially defined as
equivalent to step even though there are other commands whose
names start with s. You can test abbreviations by using them as
arguments to the help command.

A blank line as input to GDB (typing just RET) means to
repeat the previous command. Certain commands (for example, run)
will not repeat this way; these are commands whose unintentional
repetition might cause trouble and which you are unlikely to want to
repeat. User-defined commands can disable this feature; see
dont-repeat.

The list and x commands, when you repeat them with
RET, construct new arguments rather than repeating
exactly as typed. This permits easy scanning of source or memory.

GDB can also use RET in another way: to partition lengthy
output, in a way similar to the common utility more
(see Screen Size). Since it is easy to press one
RET too many in this situation, GDB disables command
repetition after any command that generates this sort of display.

Any text from a # to the end of the line is a comment; it does
nothing. This is useful mainly in command files (see Command Files).

The Ctrl-o binding is useful for repeating a complex sequence of
commands. This command accepts the current line, like RET, and
then fetches the next line relative to the current line from the history
for editing.

3.2 Command Completion

GDB can fill in the rest of a word in a command for you, if there is
only one possibility; it can also show you what the valid possibilities
are for the next word in a command, at any time. This works for GDB
commands, GDB subcommands, and the names of symbols in your program.

Press the TAB key whenever you want GDB to fill out the rest
of a word. If there is only one possibility, GDB fills in the
word, and waits for you to finish the command (or press RET to
enter it). For example, if you type

(gdb) info bre TAB

GDB fills in the rest of the word ‘breakpoints’, since that is
the only info subcommand beginning with ‘bre’:

(gdb) info breakpoints

You can either press RET at this point, to run the info
breakpoints command, or backspace and enter something else, if
‘breakpoints’ does not look like the command you expected. (If you
were sure you wanted info breakpoints in the first place, you
might as well just type RET immediately after ‘info bre’,
to exploit command abbreviations rather than command completion).

If there is more than one possibility for the next word when you press
TAB, GDB sounds a bell. You can either supply more
characters and try again, or just press TAB a second time;
GDB displays all the possible completions for that word. For
example, you might want to set a breakpoint on a subroutine whose name
begins with ‘make_’, but when you type b make_TABGDB
just sounds the bell. Typing TAB again displays all the
function names in your program that begin with those characters, for
example:

After displaying the available possibilities, GDB copies your
partial input (‘b make_’ in the example) so you can finish the
command.

If you just want to see the list of alternatives in the first place, you
can press M-? rather than pressing TAB twice. M-?
means META ?. You can type this either by holding down a
key designated as the META shift on your keyboard (if there is
one) while typing ?, or as ESC followed by ?.

If the number of possible completions is large, GDB will
print as much of the list as it has collected, as well as a message
indicating that the list may be truncated.

Set the maximum number of completion candidates. GDB will
stop looking for more completions once it collects this many candidates.
This is useful when completing on things like function names as collecting
all the possible candidates can be time consuming.
The default value is 200. A value of zero disables tab-completion.
Note that setting either no limit or a very large limit can make
completion slow.

show max-completions

Show the maximum number of candidates that GDB will collect and show
during completion.

Sometimes the string you need, while logically a “word”, may contain
parentheses or other characters that GDB normally excludes from
its notion of a word. To permit word completion to work in this
situation, you may enclose words in ' (single quote marks) in
GDB commands.

A likely situation where you might need this is in typing an
expression that involves a C++ symbol name with template
parameters. This is because when completing expressions, GDB treats
the ‘<’ character as word delimiter, assuming that it’s the
less-than comparison operator (see C and C++
Operators).

For example, when you want to call a C++ template function
interactively using the print or call commands, you may
need to distinguish whether you mean the version of name that
was specialized for int, name<int>(), or the version
that was specialized for float, name<float>(). To use
the word-completion facilities in this situation, type a single quote
' at the beginning of the function name. This alerts
GDB that it may need to consider more information than usual
when you press TAB or M-? to request word completion:

(gdb) p 'func< M-?
func<int>() func<float>()
(gdb) p 'func<

When setting breakpoints however (see Specify Location), you don’t
usually need to type a quote before the function name, because
GDB understands that you want to set a breakpoint on a
function:

(gdb) b func< M-?
func<int>() func<float>()
(gdb) b func<

This is true even in the case of typing the name of C++ overloaded
functions (multiple definitions of the same function, distinguished by
argument type). For example, when you want to set a breakpoint you
don’t need to distinguish whether you mean the version of name
that takes an int parameter, name(int), or the version
that takes a float parameter, name(float).

3.3 Getting Help

You can always ask GDB itself for information on its commands,
using the command help.

help

h

You can use help (abbreviated h) with no arguments to
display a short list of named classes of commands:

(gdb) help
List of classes of commands:
aliases -- Aliases of other commands
breakpoints -- Making program stop at certain points
data -- Examining data
files -- Specifying and examining files
internals -- Maintenance commands
obscure -- Obscure features
running -- Running the program
stack -- Examining the stack
status -- Status inquiries
support -- Support facilities
tracepoints -- Tracing of program execution without
stopping the program
user-defined -- User-defined commands
Type "help" followed by a class name for a list of
commands in that class.
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)

help class

Using one of the general help classes as an argument, you can get a
list of the individual commands in that class. For example, here is the
help display for the class status:

(gdb) help status
Status inquiries.
List of commands:
info -- Generic command for showing things
about the program being debugged
show -- Generic command for showing things
about the debugger
Type "help" followed by command name for full
documentation.
Command name abbreviations are allowed if unambiguous.
(gdb)

help command

With a command name as help argument, GDB displays a
short paragraph on how to use that command.

apropos args

The apropos command searches through all of the GDB
commands, and their documentation, for the regular expression specified in
args. It prints out all matches found. For example:

apropos alias

results in:

alias -- Define a new command that is an alias of an existing command
aliases -- Aliases of other commands
d -- Delete some breakpoints or auto-display expressions
del -- Delete some breakpoints or auto-display expressions
delete -- Delete some breakpoints or auto-display expressions

complete args

The complete args command lists all the possible completions
for the beginning of a command. Use args to specify the beginning of the
command you want completed. For example:

complete i

results in:

if
ignore
info
inspect

This is intended for use by GNU Emacs.

In addition to help, you can use the GDB commands info
and show to inquire about the state of your program, or the state
of GDB itself. Each command supports many topics of inquiry; this
manual introduces each of them in the appropriate context. The listings
under info and under show in the Command, Variable, and
Function Index point to all the sub-commands. See Command and Variable Index.

info

This command (abbreviated i) is for describing the state of your
program. For example, you can show the arguments passed to a function
with info args, list the registers currently in use with info
registers, or list the breakpoints you have set with info breakpoints.
You can get a complete list of the info sub-commands with
help info.

set

You can assign the result of an expression to an environment variable with
set. For example, you can set the GDB prompt to a $-sign with
set prompt $.

show

In contrast to info, show is for describing the state of
GDB itself.
You can change most of the things you can show, by using the
related command set; for example, you can control what number
system is used for displays with set radix, or simply inquire
which is currently in use with show radix.

To display all the settable parameters and their current
values, you can use show with no arguments; you may also use
info set. Both commands produce the same display.

Here are several miscellaneous show subcommands, all of which are
exceptional in lacking corresponding set commands:

show version

Show what version of GDB is running. You should include this
information in GDB bug-reports. If multiple versions of
GDB are in use at your site, you may need to determine which
version of GDB you are running; as GDB evolves, new
commands are introduced, and old ones may wither away. Also, many
system vendors ship variant versions of GDB, and there are
variant versions of GDB in GNU/Linux distributions as well.
The version number is the same as the one announced when you start
GDB.

show copying

info copying

Display information about permission for copying GDB.

show warranty

info warranty

Display the GNU “NO WARRANTY” statement, or a warranty,
if your version of GDB comes with one.

show configuration

Display detailed information about the way GDB was configured
when it was built. This displays the optional arguments passed to the
configure script and also configuration parameters detected
automatically by configure. When reporting a GDB
bug (see GDB Bugs), it is important to include this information in
your report.

4 Running Programs Under GDB

When you run a program under GDB, you must first generate
debugging information when you compile it.

You may start GDB with its arguments, if any, in an environment
of your choice. If you are doing native debugging, you may redirect
your program’s input and output, debug an already running process, or
kill a child process.

4.1 Compiling for Debugging

In order to debug a program effectively, you need to generate
debugging information when you compile it. This debugging information
is stored in the object file; it describes the data type of each
variable or function and the correspondence between source line numbers
and addresses in the executable code.

To request debugging information, specify the ‘-g’ option when you run
the compiler.

Programs that are to be shipped to your customers are compiled with
optimizations, using the ‘-O’ compiler option. However, some
compilers are unable to handle the ‘-g’ and ‘-O’ options
together. Using those compilers, you cannot generate optimized
executables containing debugging information.

GCC, the GNU C/C++ compiler, supports ‘-g’ with or
without ‘-O’, making it possible to debug optimized code. We
recommend that you always use ‘-g’ whenever you compile a
program. You may think your program is correct, but there is no sense
in pushing your luck. For more information, see Optimized Code.

Older versions of the GNU C compiler permitted a variant option
‘-gg’ for debugging information. GDB no longer supports this
format; if your GNU C compiler has this option, do not use it.

GDB knows about preprocessor macros and can show you their
expansion (see Macros). Most compilers do not include information
about preprocessor macros in the debugging information if you specify
the -g flag alone. Version 3.1 and later of GCC,
the GNU C compiler, provides macro information if you are using
the DWARF debugging format, and specify the option -g3.

You will have the best debugging experience if you use the latest
version of the DWARF debugging format that your compiler supports.
DWARF is currently the most expressive and best supported debugging
format in GDB.

If you are running your program in an execution environment that
supports processes, run creates an inferior process and makes
that process run your program. In some environments without processes,
run jumps to the start of your program. Other targets,
like ‘remote’, are always running. If you get an error
message like this one:

The "remote" target does not support "run".
Try "help target" or "continue".

then use continue to run your program. You may need load
first (see load).

The execution of a program is affected by certain information it
receives from its superior. GDB provides ways to specify this
information, which you must do before starting your program. (You
can change it after starting your program, but such changes only affect
your program the next time you start it.) This information may be
divided into four categories:

The arguments.

Specify the arguments to give your program as the arguments of the
run command. If a shell is available on your target, the shell
is used to pass the arguments, so that you may use normal conventions
(such as wildcard expansion or variable substitution) in describing
the arguments.
In Unix systems, you can control which shell is used with the
SHELL environment variable. If you do not define SHELL,
GDB uses the default shell (/bin/sh). You can disable
use of any shell with the set startup-with-shell command (see
below for details).

The environment.

Your program normally inherits its environment from GDB, but you can
use the GDB commands set environment and unset
environment to change parts of the environment that affect
your program. See Your Program’s Environment.

The working directory.

You can set your program’s working directory with the command
set cwd. If you do not set any working directory with this
command, your program will inherit GDB’s working directory if
native debugging, or the remote server’s working directory if remote
debugging. See Your Program’s Working
Directory.

The standard input and output.

Your program normally uses the same device for standard input and
standard output as GDB is using. You can redirect input and output
in the run command line, or you can use the tty command to
set a different device for your program.
See Your Program’s Input and Output.

Warning: While input and output redirection work, you cannot use
pipes to pass the output of the program you are debugging to another
program; if you attempt this, GDB is likely to wind up debugging the
wrong program.

When you issue the run command, your program begins to execute
immediately. See Stopping and Continuing, for discussion
of how to arrange for your program to stop. Once your program has
stopped, you may call functions in your program, using the print
or call commands. See Examining Data.

If the modification time of your symbol file has changed since the last
time GDB read its symbols, GDB discards its symbol
table, and reads it again. When it does this, GDB tries to retain
your current breakpoints.

start

The name of the main procedure can vary from language to language.
With C or C++, the main procedure name is always main, but
other languages such as Ada do not require a specific name for their
main procedure. The debugger provides a convenient way to start the
execution of the program and to stop at the beginning of the main
procedure, depending on the language used.

The ‘start’ command does the equivalent of setting a temporary
breakpoint at the beginning of the main procedure and then invoking
the ‘run’ command.

Some programs contain an elaboration phase where some startup code is
executed before the main procedure is called. This depends on the
languages used to write your program. In C++, for instance,
constructors for static and global objects are executed before
main is called. It is therefore possible that the debugger stops
before reaching the main procedure. However, the temporary breakpoint
will remain to halt execution.

Specify the arguments to give to your program as arguments to the
‘start’ command. These arguments will be given verbatim to the
underlying ‘run’ command. Note that the same arguments will be
reused if no argument is provided during subsequent calls to
‘start’ or ‘run’.

It is sometimes necessary to debug the program during elaboration. In
these cases, using the start command would stop the execution
of your program too late, as the program would have already completed
the elaboration phase. Under these circumstances, either insert
breakpoints in your elaboration code before running your program or
use the starti command.

starti

The ‘starti’ command does the equivalent of setting a temporary
breakpoint at the first instruction of a program’s execution and then
invoking the ‘run’ command. For programs containing an
elaboration phase, the starti command will stop execution at
the start of the elaboration phase.

set exec-wrapper wrapper

show exec-wrapper

unset exec-wrapper

When ‘exec-wrapper’ is set, the specified wrapper is used to
launch programs for debugging. GDB starts your program
with a shell command of the form exec wrapperprogram. Quoting is added to program and its
arguments, but not to wrapper, so you should add quotes if
appropriate for your shell. The wrapper runs until it executes
your program, and then GDB takes control.

You can use any program that eventually calls execve with
its arguments as a wrapper. Several standard Unix utilities do
this, e.g. env and nohup. Any Unix shell script ending
with exec "$@" will also work.

For example, you can use env to pass an environment variable to
the debugged program, without setting the variable in your shell’s
environment:

(gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so'
(gdb) run

This command is available when debugging locally on most targets, excluding
DJGPP, Cygwin, MS Windows, and QNX Neutrino.

set startup-with-shell

set startup-with-shell on

set startup-with-shell off

show startup-with-shell

On Unix systems, by default, if a shell is available on your target,
GDB) uses it to start your program. Arguments of the
run command are passed to the shell, which does variable
substitution, expands wildcard characters and performs redirection of
I/O. In some circumstances, it may be useful to disable such use of a
shell, for example, when debugging the shell itself or diagnosing
startup failures such as:

which indicates the shell or the wrapper specified with
‘exec-wrapper’ crashed, not your program. Most often, this is
caused by something odd in your shell’s non-interactive mode
initialization file—such as .cshrc for C-shell,
$.zshenv for the Z shell, or the file specified in the
‘BASH_ENV’ environment variable for BASH.

set auto-connect-native-target

set auto-connect-native-target on

set auto-connect-native-target off

show auto-connect-native-target

By default, if not connected to any target yet (e.g., with
target remote), the run command starts your program as a
native process under GDB, on your local machine. If you’re
sure you don’t want to debug programs on your local machine, you can
tell GDB to not connect to the native target automatically
with the set auto-connect-native-target off command.

If on, which is the default, and if GDB is not
connected to a target already, the run command automaticaly
connects to the native target, if one is available.

If off, and if GDB is not connected to a target
already, the run command fails with an error:

(gdb) run
Don't know how to run. Try "help target".

If GDB is already connected to a target, GDB always
uses it with the run command.

In any case, you can explicitly connect to the native target with the
target native command. For example,

This option (enabled by default in GDB) will turn off the native
randomization of the virtual address space of the started program. This option
is useful for multiple debugging sessions to make the execution better
reproducible and memory addresses reusable across debugging sessions.

This feature is implemented only on certain targets, including GNU/Linux.
On GNU/Linux you can get the same behavior using

(gdb) set exec-wrapper setarch `uname -m` -R

set disable-randomization off

Leave the behavior of the started executable unchanged. Some bugs rear their
ugly heads only when the program is loaded at certain addresses. If your bug
disappears when you run the program under GDB, that might be because
GDB by default disables the address randomization on platforms, such
as GNU/Linux, which do that for stand-alone programs. Use set
disable-randomization off to try to reproduce such elusive bugs.

On targets where it is available, virtual address space randomization
protects the programs against certain kinds of security attacks. In these
cases the attacker needs to know the exact location of a concrete executable
code. Randomizing its location makes it impossible to inject jumps misusing
a code at its expected addresses.

Prelinking shared libraries provides a startup performance advantage but it
makes addresses in these libraries predictable for privileged processes by
having just unprivileged access at the target system. Reading the shared
library binary gives enough information for assembling the malicious code
misusing it. Still even a prelinked shared library can get loaded at a new
random address just requiring the regular relocation process during the
startup. Shared libraries not already prelinked are always loaded at
a randomly chosen address.

Position independent executables (PIE) contain position independent code
similar to the shared libraries and therefore such executables get loaded at
a randomly chosen address upon startup. PIE executables always load even
already prelinked shared libraries at a random address. You can build such
executable using gcc -fPIE -pie.

Heap (malloc storage), stack and custom mmap areas are always placed randomly
(as long as the randomization is enabled).

show disable-randomization

Show the current setting of the explicit disable of the native randomization of
the virtual address space of the started program.

4.3 Your Program’s Arguments

The arguments to your program can be specified by the arguments of the
run command.
They are passed to a shell, which expands wildcard characters and
performs redirection of I/O, and thence to your program. Your
SHELL environment variable (if it exists) specifies what shell
GDB uses. If you do not define SHELL, GDB uses
the default shell (/bin/sh on Unix).

On non-Unix systems, the program is usually invoked directly by
GDB, which emulates I/O redirection via the appropriate system
calls, and the wildcard characters are expanded by the startup code of
the program, not by the shell.

run with no arguments uses the same arguments used by the previous
run, or those set by the set args command.

set args

Specify the arguments to be used the next time your program is run. If
set args has no arguments, run executes your program
with no arguments. Once you have run your program with arguments,
using set args before the next run is the only way to run
it again without arguments.

4.4 Your Program’s Environment

The environment consists of a set of environment variables and
their values. Environment variables conventionally record such things as
your user name, your home directory, your terminal type, and your search
path for programs to run. Usually you set up environment variables with
the shell and they are inherited by all the other programs you run. When
debugging, it can be useful to try running your program with a modified
environment without having to start GDB over again.

path directory

Add directory to the front of the PATH environment variable
(the search path for executables) that will be passed to your program.
The value of PATH used by GDB does not change.
You may specify several directory names, separated by whitespace or by a
system-dependent separator character (‘:’ on Unix, ‘;’ on
MS-DOS and MS-Windows). If directory is already in the path, it
is moved to the front, so it is searched sooner.

You can use the string ‘$cwd’ to refer to whatever is the current
working directory at the time GDB searches the path. If you
use ‘.’ instead, it refers to the directory where you executed the
path command. GDB replaces ‘.’ in the
directory argument (with the current path) before adding
directory to the search path.

show paths

Display the list of search paths for executables (the PATH
environment variable).

show environment [varname]

Print the value of environment variable varname to be given to
your program when it starts. If you do not supply varname,
print the names and values of all environment variables to be given to
your program. You can abbreviate environment as env.

set environment varname[=value]

Set environment variable varname to value. The value
changes for your program (and the shell GDB uses to launch
it), not for GDB itself. The value may be any string; the
values of environment variables are just strings, and any
interpretation is supplied by your program itself. The value
parameter is optional; if it is eliminated, the variable is set to a
null value.

For example, this command:

set env USER = foo

tells the debugged program, when subsequently run, that its user is named
‘foo’. (The spaces around ‘=’ are used for clarity here; they
are not actually required.)

Note that on Unix systems, GDB runs your program via a shell,
which also inherits the environment set with set environment.
If necessary, you can avoid that by using the ‘env’ program as a
wrapper instead of using set environment. See set exec-wrapper, for an example doing just that.

Environment variables that are set by the user are also transmitted to
gdbserver to be used when starting the remote inferior.
see QEnvironmentHexEncoded.

unset environment varname

Remove variable varname from the environment to be passed to your
program. This is different from ‘set env varname =’;
unset environment removes the variable from the environment,
rather than assigning it an empty value.

Environment variables that are unset by the user are also unset on
gdbserver when starting the remote inferior.
see QEnvironmentUnset.

Warning: On Unix systems, GDB runs your program using
the shell indicated by your SHELL environment variable if it
exists (or /bin/sh if not). If your SHELL variable
names a shell that runs an initialization file when started
non-interactively—such as .cshrc for C-shell, $.zshenv
for the Z shell, or the file specified in the ‘BASH_ENV’
environment variable for BASH—any variables you set in that file
affect your program. You may wish to move setting of environment
variables to files that are only run when you sign on, such as
.login or .profile.

4.5 Your Program’s Working Directory

Each time you start your program with run, the inferior will be
initialized with the current working directory specified by the
set cwd command. If no directory has been specified by this
command, then the inferior will inherit GDB’s current working
directory as its working directory if native debugging, or it will
inherit the remote server’s current working directory if remote
debugging.

set cwd [directory]

Set the inferior’s working directory to directory, which will be
glob-expanded in order to resolve tildes (~). If no
argument has been specified, the command clears the setting and resets
it to an empty state. This setting has no effect on GDB’s
working directory, and it only takes effect the next time you start
the inferior. The ~ in directory is a short for the
home directory, usually pointed to by the HOME environment
variable. On MS-Windows, if HOME is not defined, GDB
uses the concatenation of HOMEDRIVE and HOMEPATH as
fallback.

You can also change GDB’s current working directory by using
the cd command.
See cd command.

show cwd

Show the inferior’s working directory. If no directory has been
specified by set cwd, then the default inferior’s working
directory is the same as GDB’s working directory.

cd [directory]

Set the GDB working directory to directory. If not
given, directory uses '~'.

It is generally impossible to find the current working directory of
the process being debugged (since a program can change its directory
during its run). If you work on a system where GDB supports
the info proc command (see Process Information), you can
use the info proc command to find out the
current working directory of the debuggee.

4.6 Your Program’s Input and Output

By default, the program you run under GDB does input and output to
the same terminal that GDB uses. GDB switches the terminal
to its own terminal modes to interact with you, but it records the terminal
modes your program was using and switches back to them when you continue
running your program.

info terminal

Displays information recorded by GDB about the terminal modes your
program is using.

You can redirect your program’s input and/or output using shell
redirection with the run command. For example,

run > outfile

starts your program, diverting its output to the file outfile.

Another way to specify where your program should do input and output is
with the tty command. This command accepts a file name as
argument, and causes this file to be the default for future run
commands. It also resets the controlling terminal for the child
process, for future run commands. For example,

tty /dev/ttyb

directs that processes started with subsequent run commands
default to do input and output on the terminal /dev/ttyb and have
that as their controlling terminal.

An explicit redirection in run overrides the tty command’s
effect on the input/output device, but not its effect on the controlling
terminal.

When you use the tty command or redirect input in the run
command, only the input for your program is affected. The input
for GDB still comes from your terminal. tty is an alias
for set inferior-tty.

You can use the show inferior-tty command to tell GDB to
display the name of the terminal that will be used for future runs of your
program.

set inferior-tty [ tty ]

Set the tty for the program being debugged to tty. Omitting tty
restores the default behavior, which is to use the same terminal as
GDB.

4.7 Debugging an Already-running Process

attach process-id

This command attaches to a running process—one that was started
outside GDB. (info files shows your active
targets.) The command takes as argument a process ID. The usual way to
find out the process-id of a Unix process is with the ps utility,
or with the ‘jobs -l’ shell command.

attach does not repeat if you press RET a second time after
executing the command.

To use attach, your program must be running in an environment
which supports processes; for example, attach does not work for
programs on bare-board targets that lack an operating system. You must
also have permission to send the process a signal.

When you use attach, the debugger finds the program running in
the process first by looking in the current working directory, then (if
the program is not found) by using the source file search path
(see Specifying Source Directories). You can also use
the file command to load the program. See Commands to
Specify Files.

The first thing GDB does after arranging to debug the specified
process is to stop it. You can examine and modify an attached process
with all the GDB commands that are ordinarily available when
you start processes with run. You can insert breakpoints; you
can step and continue; you can modify storage. If you would rather the
process continue running, you may use the continue command after
attaching GDB to the process.

detach

When you have finished debugging the attached process, you can use the
detach command to release it from GDB control. Detaching
the process continues its execution. After the detach command,
that process and GDB become completely independent once more, and you
are ready to attach another process or start one with run.
detach does not repeat if you press RET again after
executing the command.

If you exit GDB while you have an attached process, you detach
that process. If you use the run command, you kill that process.
By default, GDB asks for confirmation if you try to do either of these
things; you can control whether or not you need to confirm by using the
set confirm command (see Optional Warnings and
Messages).

4.8 Killing the Child Process

This command is useful if you wish to debug a core dump instead of a
running process. GDB ignores any core dump file while your program
is running.

On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB. You can use the
kill command in this situation to permit running your program
outside the debugger.

The kill command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process. In this case, when you
next type run, GDB notices that the file has changed, and
reads the symbol table again (while trying to preserve your current
breakpoint settings).

4.9 Debugging Multiple Inferiors and Programs

GDB lets you run and debug multiple programs in a single
session. In addition, GDB on some systems may let you run
several programs simultaneously (otherwise you have to exit from one
before starting another). In the most general case, you can have
multiple threads of execution in each of multiple processes, launched
from multiple executables.

GDB represents the state of each program execution with an
object called an inferior. An inferior typically corresponds to
a process, but is more general and applies also to targets that do not
have processes. Inferiors may be created before a process runs, and
may be retained after a process exits. Inferiors have unique
identifiers that are different from process ids. Usually each
inferior will also have its own distinct address space, although some
embedded targets may have several inferiors running in different parts
of a single address space. Each inferior may in turn have multiple
threads running in it.

To find out what inferiors exist at any moment, use info inferiors:

info inferiors

Print a list of all inferiors currently being managed by GDB.
By default all inferiors are printed, but the argument id…
– a space separated list of inferior numbers – can be used to limit
the display to just the requested inferiors.

GDB displays for each inferior (in this order):

the inferior number assigned by GDB

the target system’s inferior identifier

the name of the executable the inferior is running.

An asterisk ‘*’ preceding the GDB inferior number
indicates the current inferior.

Make inferior number infno the current inferior. The argument
infno is the inferior number assigned by GDB, as shown
in the first field of the ‘info inferiors’ display.

The debugger convenience variable ‘$_inferior’ contains the
number of the current inferior. You may find this useful in writing
breakpoint conditional expressions, command scripts, and so forth.
See Convenience Variables, for general
information on convenience variables.

You can get multiple executables into a debugging session via the
add-inferior and clone-inferior commands. On some
systems GDB can add inferiors to the debug session
automatically by following calls to fork and exec. To
remove inferiors from the debugging session use the
remove-inferiors command.

add-inferior [ -copies n ] [ -exec executable ]

Adds n inferiors to be run using executable as the
executable; n defaults to 1. If no executable is specified,
the inferiors begins empty, with no program. You can still assign or
change the program assigned to the inferior at any time by using the
file command with the executable name as its argument.

clone-inferior [ -copies n ] [ infno ]

Adds n inferiors ready to execute the same program as inferior
infno; n defaults to 1, and infno defaults to the
number of the current inferior. This is a convenient command when you
want to run another instance of the inferior you are debugging.

Removes the inferior or inferiors infno…. It is not
possible to remove an inferior that is running with this command. For
those, use the kill or detach command first.

To quit debugging one of the running inferiors that is not the current
inferior, you can either detach from it by using the detach inferior command (allowing it to run independently), or kill it
using the kill inferiors command:

detach inferior infno…

Detach from the inferior or inferiors identified by GDB
inferior number(s) infno…. Note that the inferior’s entry
still stays on the list of inferiors shown by info inferiors,
but its Description will show ‘<null>’.

kill inferiors infno…

Kill the inferior or inferiors identified by GDB inferior
number(s) infno…. Note that the inferior’s entry still
stays on the list of inferiors shown by info inferiors, but its
Description will show ‘<null>’.

After the successful completion of a command such as detach,
detach inferiors, kill or kill inferiors, or after
a normal process exit, the inferior is still valid and listed with
info inferiors, ready to be restarted.

To be notified when inferiors are started or exit under GDB’s
control use set print inferior-events:

set print inferior-events

set print inferior-events on

set print inferior-events off

The set print inferior-events command allows you to enable or
disable printing of messages when GDB notices that new
inferiors have started or that inferiors have exited or have been
detached. By default, these messages will not be printed.

show print inferior-events

Show whether messages will be printed when GDB detects that
inferiors have started, exited or have been detached.

Many commands will work the same with multiple programs as with a
single program: e.g., print myglobal will simply display the
value of myglobal in the current inferior.

Occasionaly, when debugging GDB itself, it may be useful to
get more info about the relationship of inferiors, programs, address
spaces in a debug session. You can do that with the maint info program-spaces command.

maint info program-spaces

Print a list of all program spaces currently being managed by
GDB.

GDB displays for each program space (in this order):

the program space number assigned by GDB

the name of the executable loaded into the program space, with e.g.,
the file command.

An asterisk ‘*’ preceding the GDB program space number
indicates the current program space.

In addition, below each program space line, GDB prints extra
information that isn’t suitable to display in tabular form. For
example, the list of inferiors bound to the program space.

Here we can see that no inferior is running the program hello,
while process 21561 is running the program goodbye. On
some targets, it is possible that multiple inferiors are bound to the
same program space. The most common example is that of debugging both
the parent and child processes of a vfork call. For example,

4.10 Debugging Programs with Multiple Threads

In some operating systems, such as GNU/Linux and Solaris, a single program
may have more than one thread of execution. The precise semantics
of threads differ from one operating system to another, but in general
the threads of a single program are akin to multiple processes—except
that they share one address space (that is, they can all examine and
modify the same variables). On the other hand, each thread has its own
registers and execution stack, and perhaps private memory.

GDB provides these facilities for debugging multi-thread
programs:

automatic notification of new threads

‘thread thread-id’, a command to switch among threads

‘info threads’, a command to inquire about existing threads

‘thread apply [thread-id-list] [all] args’,
a command to apply a command to a list of threads

thread-specific breakpoints

‘set print thread-events’, which controls printing of
messages on thread start and exit.

‘set libthread-db-search-path path’, which lets
the user specify which libthread_db to use if the default choice
isn’t compatible with the program.

The GDB thread debugging facility allows you to observe all
threads while your program runs—but whenever GDB takes
control, one thread in particular is always the focus of debugging.
This thread is called the current thread. Debugging commands show
program information from the perspective of the current thread.

Whenever GDB detects a new thread in your program, it displays
the target system’s identification for the thread with a message in the
form ‘[New systag]’, where systag is a thread identifier
whose form varies depending on the particular system. For example, on
GNU/Linux, you might see

[New Thread 0x41e02940 (LWP 25582)]

when GDB notices a new thread. In contrast, on other systems,
the systag is simply something like ‘process 368’, with no
further qualifier.

For debugging purposes, GDB associates its own thread number
—always a single integer—with each thread of an inferior. This
number is unique between all threads of an inferior, but not unique
between threads of different inferiors.

You can refer to a given thread in an inferior using the qualified
inferior-num.thread-num syntax, also known as
qualified thread ID, with inferior-num being the inferior
number and thread-num being the thread number of the given
inferior. For example, thread 2.3 refers to thread number 3 of
inferior 2. If you omit inferior-num (e.g., thread 3),
then GDB infers you’re referring to a thread of the current
inferior.

Until you create a second inferior, GDB does not show the
inferior-num part of thread IDs, even though you can always use
the full inferior-num.thread-num form to refer to threads
of inferior 1, the initial inferior.

Some commands accept a space-separated thread ID list as
argument. A list element can be:

A thread ID as shown in the first field of the ‘info threads’
display, with or without an inferior qualifier. E.g., ‘2.1’ or
‘1’.

A range of thread numbers, again with or without an inferior
qualifier, as in inf.thr1-thr2 or
thr1-thr2. E.g., ‘1.2-4’ or ‘2-4’.

All threads of an inferior, specified with a star wildcard, with or
without an inferior qualifier, as in inf.* (e.g.,
‘1.*’) or *. The former refers to all threads of the
given inferior, and the latter form without an inferior qualifier
refers to all threads of the current inferior.

For example, if the current inferior is 1, and inferior 7 has one
thread with ID 7.1, the thread list ‘1 2-3 4.5 6.7-9 7.*’
includes threads 1 to 3 of inferior 1, thread 5 of inferior 4, threads
7 to 9 of inferior 6 and all threads of inferior 7. That is, in
expanded qualified form, the same as ‘1.1 1.2 1.3 4.5 6.7 6.8 6.9
7.1’.

In addition to a per-inferior number, each thread is also
assigned a unique global number, also known as global
thread ID, a single integer. Unlike the thread number component of
the thread ID, no two threads have the same global ID, even when
you’re debugging multiple inferiors.

From GDB’s perspective, a process always has at least one
thread. In other words, GDB assigns a thread number to the
program’s “main thread” even if the program is not multi-threaded.

The debugger convenience variables ‘$_thread’ and
‘$_gthread’ contain, respectively, the per-inferior thread number
and the global thread number of the current thread. You may find this
useful in writing breakpoint conditional expressions, command scripts,
and so forth. See Convenience Variables, for
general information on convenience variables.

If GDB detects the program is multi-threaded, it augments the
usual message about stopping at a breakpoint with the ID and name of
the thread that hit the breakpoint.

Thread 2 "client" hit Breakpoint 1, send_message () at client.c:68

Likewise when the program receives a signal:

Thread 1 "main" received signal SIGINT, Interrupt.

info threads [thread-id-list]

Display information about one or more threads. With no arguments
displays information about all threads. You can specify the list of
threads that you want to display using the thread ID list syntax
(see thread ID lists).

GDB displays for each thread (in this order):

the per-inferior thread number assigned by GDB

the global thread number assigned by GDB, if the ‘-gid’
option was specified

the target system’s thread identifier (systag)

the thread’s name, if one is known. A thread can either be named by
the user (see thread name, below), or, in some cases, by the
program itself.

the current stack frame summary for that thread

An asterisk ‘*’ to the left of the GDB thread number
indicates the current thread.

On Solaris, you can display more information about user threads with a
Solaris-specific command:

maint info sol-threads

Display info on Solaris user threads.

thread thread-id

Make thread ID thread-id the current thread. The command
argument thread-id is the GDB thread ID, as shown in
the first field of the ‘info threads’ display, with or without an
inferior qualifier (e.g., ‘2.1’ or ‘1’).

GDB responds by displaying the system identifier of the
thread you selected, and its current stack frame summary:

As with the ‘[New …]’ message, the form of the text after
‘Switching to’ depends on your system’s conventions for identifying
threads.

thread apply [thread-id-list | all [-ascending]] command

The thread apply command allows you to apply the named
command to one or more threads. Specify the threads that you
want affected using the thread ID list syntax (see thread ID lists), or specify all to apply to all threads. To apply a
command to all threads in descending order, type thread apply all
command. To apply a command to all threads in ascending order,
type thread apply all -ascending command.

thread name [name]

This command assigns a name to the current thread. If no argument is
given, any existing user-specified name is removed. The thread name
appears in the ‘info threads’ display.

On some systems, such as GNU/Linux, GDB is able to
determine the name of the thread as given by the OS. On these
systems, a name specified with ‘thread name’ will override the
system-give name, and removing the user-specified name will cause
GDB to once again display the system-specified name.

thread find [regexp]

Search for and display thread ids whose name or systag
matches the supplied regular expression.

As well as being the complement to the ‘thread name’ command,
this command also allows you to identify a thread by its target
systag. For instance, on GNU/Linux, the target systag
is the LWP id.

The set print thread-events command allows you to enable or
disable printing of messages when GDB notices that new threads have
started or that threads have exited. By default, these messages will
be printed if detection of these events is supported by the target.
Note that these messages cannot be disabled on all targets.

show print thread-events

Show whether messages will be printed when GDB detects that threads
have started and exited.

If this variable is set, path is a colon-separated list of
directories GDB will use to search for libthread_db.
If you omit path, ‘libthread-db-search-path’ will be reset to
its default value ($sdir:$pdir on GNU/Linux and Solaris systems).
Internally, the default value comes from the LIBTHREAD_DB_SEARCH_PATH
macro.

On GNU/Linux and Solaris systems, GDB uses a “helper”
libthread_db library to obtain information about threads in the
inferior process. GDB will use ‘libthread-db-search-path’
to find libthread_db. GDB also consults first if inferior
specific thread debugging library loading is enabled
by ‘set auto-load libthread-db’ (see libthread_db.so.1 file).

A special entry ‘$sdir’ for ‘libthread-db-search-path’
refers to the default system directories that are
normally searched for loading shared libraries. The ‘$sdir’ entry
is the only kind not needing to be enabled by ‘set auto-load libthread-db’
(see libthread_db.so.1 file).

A special entry ‘$pdir’ for ‘libthread-db-search-path’
refers to the directory from which libpthread
was loaded in the inferior process.

For any libthread_db library GDB finds in above directories,
GDB attempts to initialize it with the current inferior process.
If this initialization fails (which could happen because of a version
mismatch between libthread_db and libpthread), GDB
will unload libthread_db, and continue with the next directory.
If none of libthread_db libraries initialize successfully,
GDB will issue a warning and thread debugging will be disabled.

Setting libthread-db-search-path is currently implemented
only on some platforms.

show libthread-db-search-path

Display current libthread_db search path.

set debug libthread-db

show debug libthread-db

Turns on or off display of libthread_db-related events.
Use 1 to enable, 0 to disable.

4.11 Debugging Forks

On most systems, GDB has no special support for debugging
programs which create additional processes using the fork
function. When a program forks, GDB will continue to debug the
parent process and the child process will run unimpeded. If you have
set a breakpoint in any code which the child then executes, the child
will get a SIGTRAP signal which (unless it catches the signal)
will cause it to terminate.

However, if you want to debug the child process there is a workaround
which isn’t too painful. Put a call to sleep in the code which
the child process executes after the fork. It may be useful to sleep
only if a certain environment variable is set, or a certain file exists,
so that the delay need not occur when you don’t want to run GDB
on the child. While the child is sleeping, use the ps program to
get its process ID. Then tell GDB (a new invocation of
GDB if you are also debugging the parent process) to attach to
the child process (see Attach). From that point on you can debug
the child process just like any other process which you attached to.

On some systems, GDB provides support for debugging programs
that create additional processes using the fork or vfork
functions. On GNU/Linux platforms, this feature is supported
with kernel version 2.5.46 and later.

The fork debugging commands are supported in native mode and when
connected to gdbserver in either target remote mode or
target extended-remote mode.

By default, when a program forks, GDB will continue to debug
the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process,
use the command set follow-fork-mode.

set follow-fork-mode mode

Set the debugger response to a program call of fork or
vfork. A call to fork or vfork creates a new
process. The mode argument can be:

parent

The original process is debugged after a fork. The child process runs
unimpeded. This is the default.

child

The new process is debugged after a fork. The parent process runs
unimpeded.

show follow-fork-mode

Display the current debugger response to a fork or vfork call.

On Linux, if you want to debug both the parent and child processes, use the
command set detach-on-fork.

set detach-on-fork mode

Tells gdb whether to detach one of the processes after a fork, or
retain debugger control over them both.

on

The child process (or parent process, depending on the value of
follow-fork-mode) will be detached and allowed to run
independently. This is the default.

off

Both processes will be held under the control of GDB.
One process (child or parent, depending on the value of
follow-fork-mode) is debugged as usual, while the other
is held suspended.

show detach-on-fork

Show whether detach-on-fork mode is on/off.

If you choose to set ‘detach-on-fork’ mode off, then GDB
will retain control of all forked processes (including nested forks).
You can list the forked processes under the control of GDB by
using the info inferiors command, and switch from one fork
to another by using the inferior command (see Debugging Multiple Inferiors and Programs).

To quit debugging one of the forked processes, you can either detach
from it by using the detach inferiors command (allowing it
to run independently), or kill it using the kill inferiors
command. See Debugging Multiple Inferiors
and Programs.

If you ask to debug a child process and a vfork is followed by an
exec, GDB executes the new target up to the first
breakpoint in the new target. If you have a breakpoint set on
main in your original program, the breakpoint will also be set on
the child process’s main.

On some systems, when a child process is spawned by vfork, you
cannot debug the child or parent until an exec call completes.

If you issue a run command to GDB after an exec
call executes, the new target restarts. To restart the parent
process, use the file command with the parent executable name
as its argument. By default, after an exec call executes,
GDB discards the symbols of the previous executable image.
You can change this behaviour with the set follow-exec-mode
command.

set follow-exec-mode mode

Set debugger response to a program call of exec. An
exec call replaces the program image of a process.

follow-exec-mode can be:

new

GDB creates a new inferior and rebinds the process to this
new inferior. The program the process was running before the
exec call can be restarted afterwards by restarting the
original inferior.

GDB keeps the process bound to the same inferior. The new
executable image replaces the previous executable loaded in the
inferior. Restarting the inferior after the exec call, with
e.g., the run command, restarts the executable the process was
running after the exec call. This is the default mode.

4.12 Setting a Bookmark to Return to Later

On certain operating systems3, GDB is able to save a snapshot of a
program’s state, called a checkpoint, and come back to it
later.

Returning to a checkpoint effectively undoes everything that has
happened in the program since the checkpoint was saved. This
includes changes in memory, registers, and even (within some limits)
system state. Effectively, it is like going back in time to the
moment when the checkpoint was saved.

Thus, if you’re stepping thru a program and you think you’re
getting close to the point where things go wrong, you can save
a checkpoint. Then, if you accidentally go too far and miss
the critical statement, instead of having to restart your program
from the beginning, you can just go back to the checkpoint and
start again from there.

This can be especially useful if it takes a lot of time or
steps to reach the point where you think the bug occurs.

To use the checkpoint/restart method of debugging:

checkpoint

Save a snapshot of the debugged program’s current execution state.
The checkpoint command takes no arguments, but each checkpoint
is assigned a small integer id, similar to a breakpoint id.

info checkpoints

List the checkpoints that have been saved in the current debugging
session. For each checkpoint, the following information will be
listed:

Checkpoint ID

Process ID

Code Address

Source line, or label

restart checkpoint-id

Restore the program state that was saved as checkpoint number
checkpoint-id. All program variables, registers, stack frames
etc. will be returned to the values that they had when the checkpoint
was saved. In essence, gdb will “wind back the clock” to the point
in time when the checkpoint was saved.

Note that breakpoints, GDB variables, command history etc.
are not affected by restoring a checkpoint. In general, a checkpoint
only restores things that reside in the program being debugged, not in
the debugger.

delete checkpoint checkpoint-id

Delete the previously-saved checkpoint identified by checkpoint-id.

Returning to a previously saved checkpoint will restore the user state
of the program being debugged, plus a significant subset of the system
(OS) state, including file pointers. It won’t “un-write” data from
a file, but it will rewind the file pointer to the previous location,
so that the previously written data can be overwritten. For files
opened in read mode, the pointer will also be restored so that the
previously read data can be read again.

Of course, characters that have been sent to a printer (or other
external device) cannot be “snatched back”, and characters received
from eg. a serial device can be removed from internal program buffers,
but they cannot be “pushed back” into the serial pipeline, ready to
be received again. Similarly, the actual contents of files that have
been changed cannot be restored (at this time).

However, within those constraints, you actually can “rewind” your
program to a previously saved point in time, and begin debugging it
again — and you can change the course of events so as to debug a
different execution path this time.

Finally, there is one bit of internal program state that will be
different when you return to a checkpoint — the program’s process
id. Each checkpoint will have a unique process id (or pid),
and each will be different from the program’s original pid.
If your program has saved a local copy of its process id, this could
potentially pose a problem.

4.12.1 A Non-obvious Benefit of Using Checkpoints

On some systems such as GNU/Linux, address space randomization
is performed on new processes for security reasons. This makes it
difficult or impossible to set a breakpoint, or watchpoint, on an
absolute address if you have to restart the program, since the
absolute location of a symbol will change from one execution to the
next.

A checkpoint, however, is an identical copy of a process.
Therefore if you create a checkpoint at (eg.) the start of main,
and simply return to that checkpoint instead of restarting the
process, you can avoid the effects of address randomization and
your symbols will all stay in the same place.

5 Stopping and Continuing

The principal purposes of using a debugger are so that you can stop your
program before it terminates; or so that, if your program runs into
trouble, you can investigate and find out why.

Inside GDB, your program may stop for any of several reasons,
such as a signal, a breakpoint, or reaching a new line after a
GDB command such as step. You may then examine and
change variables, set new breakpoints or remove old ones, and then
continue execution. Usually, the messages shown by GDB provide
ample explanation of the status of your program—but you can also
explicitly request this information at any time.

info program

Display information about the status of your program: whether it is
running or not, what process it is, and why it stopped.

5.1 Breakpoints, Watchpoints, and Catchpoints

A breakpoint makes your program stop whenever a certain point in
the program is reached. For each breakpoint, you can add conditions to
control in finer detail whether your program stops. You can set
breakpoints with the break command and its variants (see Setting Breakpoints), to specify the place where your program
should stop by line number, function name or exact address in the
program.

On some systems, you can set breakpoints in shared libraries before
the executable is run.

A watchpoint is a special breakpoint that stops your program
when the value of an expression changes. The expression may be a value
of a variable, or it could involve values of one or more variables
combined by operators, such as ‘a + b’. This is sometimes called
data breakpoints. You must use a different command to set
watchpoints (see Setting Watchpoints), but aside
from that, you can manage a watchpoint like any other breakpoint: you
enable, disable, and delete both breakpoints and watchpoints using the
same commands.

You can arrange to have values from your program displayed automatically
whenever GDB stops at a breakpoint. See Automatic Display.

A catchpoint is another special breakpoint that stops your program
when a certain kind of event occurs, such as the throwing of a C++
exception or the loading of a library. As with watchpoints, you use a
different command to set a catchpoint (see Setting
Catchpoints), but aside from that, you can manage a catchpoint like any
other breakpoint. (To stop when your program receives a signal, use the
handle command; see Signals.)

GDB assigns a number to each breakpoint, watchpoint, or
catchpoint when you create it; these numbers are successive integers
starting with one. In many of the commands for controlling various
features of breakpoints you use the breakpoint number to say which
breakpoint you want to change. Each breakpoint may be enabled or
disabled; if disabled, it has no effect on your program until you
enable it again.

Some GDB commands accept a space-separated list of breakpoints
on which to operate. A list element can be either a single breakpoint number,
like ‘5’, or a range of such numbers, like ‘5-7’.
When a breakpoint list is given to a command, all breakpoints in that list
are operated on.

5.1.1 Setting Breakpoints

Breakpoints are set with the break command (abbreviated
b). The debugger convenience variable ‘$bpnum’ records the
number of the breakpoint you’ve set most recently; see Convenience Variables, for a discussion of what you can do with
convenience variables.

break location

Set a breakpoint at the given location, which can specify a
function name, a line number, or an address of an instruction.
(See Specify Location, for a list of all the possible ways to
specify a location.) The breakpoint will stop your program just
before it executes any of the code in the specified location.

When using source languages that permit overloading of symbols, such as
C++, a function name may refer to more than one possible place to break.
See Ambiguous Expressions, for a discussion of
that situation.

It is also possible to insert a breakpoint that will stop the program
only if a specific thread (see Thread-Specific Breakpoints)
or a specific task (see Ada Tasks) hits that breakpoint.

break

When called without any arguments, break sets a breakpoint at
the next instruction to be executed in the selected stack frame
(see Examining the Stack). In any selected frame but the
innermost, this makes your program stop as soon as control
returns to that frame. This is similar to the effect of a
finish command in the frame inside the selected frame—except
that finish does not leave an active breakpoint. If you use
break without an argument in the innermost frame, GDB stops
the next time it reaches the current location; this may be useful
inside loops.

GDB normally ignores breakpoints when it resumes execution, until at
least one instruction has been executed. If it did not do this, you
would be unable to proceed past a breakpoint without first disabling the
breakpoint. This rule applies whether or not the breakpoint already
existed when your program stopped.

break … if cond

Set a breakpoint with condition cond; evaluate the expression
cond each time the breakpoint is reached, and stop only if the
value is nonzero—that is, if cond evaluates as true.
‘…’ stands for one of the possible arguments described
above (or no argument) specifying where to break. See Break Conditions, for more information on breakpoint conditions.

tbreak args

Set a breakpoint enabled only for one stop. The args are the
same as for the break command, and the breakpoint is set in the same
way, but the breakpoint is automatically deleted after the first time your
program stops there. See Disabling Breakpoints.

hbreak args

Set a hardware-assisted breakpoint. The args are the same as for the
break command and the breakpoint is set in the same way, but the
breakpoint requires hardware support and some target hardware may not
have this support. The main purpose of this is EPROM/ROM code
debugging, so you can set a breakpoint at an instruction without
changing the instruction. This can be used with the new trap-generation
provided by SPARClite DSU and most x86-based targets. These targets
will generate traps when a program accesses some data or instruction
address that is assigned to the debug registers. However the hardware
breakpoint registers can take a limited number of breakpoints. For
example, on the DSU, only two data breakpoints can be set at a time, and
GDB will reject this command if more than two are used. Delete
or disable unused hardware breakpoints before setting new ones
(see Disabling Breakpoints).
See Break Conditions.
For remote targets, you can restrict the number of hardware
breakpoints GDB will use, see set remote hardware-breakpoint-limit.

thbreak args

Set a hardware-assisted breakpoint enabled only for one stop. The args
are the same as for the hbreak command and the breakpoint is set in
the same way. However, like the tbreak command,
the breakpoint is automatically deleted after the
first time your program stops there. Also, like the hbreak
command, the breakpoint requires hardware support and some target hardware
may not have this support. See Disabling Breakpoints.
See also Break Conditions.

rbreak regex

Set breakpoints on all functions matching the regular expression
regex. This command sets an unconditional breakpoint on all
matches, printing a list of all breakpoints it set. Once these
breakpoints are set, they are treated just like the breakpoints set with
the break command. You can delete them, disable them, or make
them conditional the same way as any other breakpoint.

The syntax of the regular expression is the standard one used with tools
like grep. Note that this is different from the syntax used by
shells, so for instance foo* matches all functions that include
an fo followed by zero or more os. There is an implicit
.* leading and trailing the regular expression you supply, so to
match only functions that begin with foo, use ^foo.

When debugging C++ programs, rbreak is useful for setting
breakpoints on overloaded functions that are not members of any special
classes.

The rbreak command can be used to set breakpoints in
all the functions in a program, like this:

(gdb) rbreak .

rbreak file:regex

If rbreak is called with a filename qualification, it limits
the search for functions matching the given regular expression to the
specified file. This can be used, for example, to set breakpoints on
every function in a given file:

(gdb) rbreak file.c:.

The colon separating the filename qualifier from the regex may
optionally be surrounded by spaces.

info breakpoints [list…]

info break [list…]

Print a table of all breakpoints, watchpoints, and catchpoints set and
not deleted. Optional argument n means print information only
about the specified breakpoint(s) (or watchpoint(s) or catchpoint(s)).
For each breakpoint, following columns are printed:

Breakpoint Numbers

Type

Breakpoint, watchpoint, or catchpoint.

Disposition

Whether the breakpoint is marked to be disabled or deleted when hit.

Enabled or Disabled

Enabled breakpoints are marked with ‘y’. ‘n’ marks breakpoints
that are not enabled.

Address

Where the breakpoint is in your program, as a memory address. For a
pending breakpoint whose address is not yet known, this field will
contain ‘<PENDING>’. Such breakpoint won’t fire until a shared
library that has the symbol or line referred by breakpoint is loaded.
See below for details. A breakpoint with several locations will
have ‘<MULTIPLE>’ in this field—see below for details.

What

Where the breakpoint is in the source for your program, as a file and
line number. For a pending breakpoint, the original string passed to
the breakpoint command will be listed as it cannot be resolved until
the appropriate shared library is loaded in the future.

If a breakpoint is conditional, there are two evaluation modes: “host” and
“target”. If mode is “host”, breakpoint condition evaluation is done by
GDB on the host’s side. If it is “target”, then the condition
is evaluated by the target. The info break command shows
the condition on the line following the affected breakpoint, together with
its condition evaluation mode in between parentheses.

Breakpoint commands, if any, are listed after that. A pending breakpoint is
allowed to have a condition specified for it. The condition is not parsed for
validity until a shared library is loaded that allows the pending
breakpoint to resolve to a valid location.

info break with a breakpoint
number n as argument lists only that breakpoint. The
convenience variable $_ and the default examining-address for
the x command are set to the address of the last breakpoint
listed (see Examining Memory).

info break displays a count of the number of times the breakpoint
has been hit. This is especially useful in conjunction with the
ignore command. You can ignore a large number of breakpoint
hits, look at the breakpoint info to see how many times the breakpoint
was hit, and then run again, ignoring one less than that number. This
will get you quickly to the last hit of that breakpoint.

For a breakpoints with an enable count (xref) greater than 1,
info break also displays that count.

GDB allows you to set any number of breakpoints at the same place in
your program. There is nothing silly or meaningless about this. When
the breakpoints are conditional, this is even useful
(see Break Conditions).

It is possible that a breakpoint corresponds to several locations
in your program. Examples of this situation are:

Multiple functions in the program may have the same name.

For a C++ constructor, the GCC compiler generates several
instances of the function body, used in different cases.

For a C++ template function, a given line in the function can
correspond to any number of instantiations.

For an inlined function, a given source line can correspond to
several places where that function is inlined.

In all those cases, GDB will insert a breakpoint at all
the relevant locations.

A breakpoint with multiple locations is displayed in the breakpoint
table using several rows—one header row, followed by one row for
each breakpoint location. The header row has ‘<MULTIPLE>’ in the
address column. The rows for individual locations contain the actual
addresses for locations, and show the functions to which those
locations belong. The number column for a location is of the form
breakpoint-number.location-number.

You cannot delete the individual locations from a breakpoint. However,
each location can be individually enabled or disabled by passing
breakpoint-number.location-number as argument to the
enable and disable commands. It’s also possible to
enable and disable a range of location-number
locations using a breakpoint-number and two location-numbers,
in increasing order, separated by a hyphen, like
breakpoint-number.location-number1-location-number2,
in which case GDB acts on all the locations in the range (inclusive).
Disabling or enabling the parent breakpoint (see Disabling) affects
all of the locations that belong to that breakpoint.

It’s quite common to have a breakpoint inside a shared library.
Shared libraries can be loaded and unloaded explicitly,
and possibly repeatedly, as the program is executed. To support
this use case, GDB updates breakpoint locations whenever
any shared library is loaded or unloaded. Typically, you would
set a breakpoint in a shared library at the beginning of your
debugging session, when the library is not loaded, and when the
symbols from the library are not available. When you try to set
breakpoint, GDB will ask you if you want to set
a so called pending breakpoint—breakpoint whose address
is not yet resolved.

After the program is run, whenever a new shared library is loaded,
GDB reevaluates all the breakpoints. When a newly loaded
shared library contains the symbol or line referred to by some
pending breakpoint, that breakpoint is resolved and becomes an
ordinary breakpoint. When a library is unloaded, all breakpoints
that refer to its symbols or source lines become pending again.

This logic works for breakpoints with multiple locations, too. For
example, if you have a breakpoint in a C++ template function, and
a newly loaded shared library has an instantiation of that template,
a new location is added to the list of locations for the breakpoint.

Except for having unresolved address, pending breakpoints do not
differ from regular breakpoints. You can set conditions or commands,
enable and disable them and perform other breakpoint operations.

GDB provides some additional commands for controlling what
happens when the ‘break’ command cannot resolve breakpoint
address specification to an address:

set breakpoint pending auto

This is the default behavior. When GDB cannot find the breakpoint
location, it queries you whether a pending breakpoint should be created.

set breakpoint pending on

This indicates that an unrecognized breakpoint location should automatically
result in a pending breakpoint being created.

set breakpoint pending off

This indicates that pending breakpoints are not to be created. Any
unrecognized breakpoint location results in an error. This setting does
not affect any pending breakpoints previously created.

show breakpoint pending

Show the current behavior setting for creating pending breakpoints.

The settings above only affect the break command and its
variants. Once breakpoint is set, it will be automatically updated
as shared libraries are loaded and unloaded.

For some targets, GDB can automatically decide if hardware or
software breakpoints should be used, depending on whether the
breakpoint address is read-only or read-write. This applies to
breakpoints set with the break command as well as to internal
breakpoints set by commands like next and finish. For
breakpoints set with hbreak, GDB will always use hardware
breakpoints.

You can control this automatic behaviour with the following commands:

set breakpoint auto-hw on

This is the default behavior. When GDB sets a breakpoint, it
will try to use the target memory map to decide if software or hardware
breakpoint must be used.

set breakpoint auto-hw off

This indicates GDB should not automatically select breakpoint
type. If the target provides a memory map, GDB will warn when
trying to set software breakpoint at a read-only address.

GDB normally implements breakpoints by replacing the program code
at the breakpoint address with a special instruction, which, when
executed, given control to the debugger. By default, the program
code is so modified only when the program is resumed. As soon as
the program stops, GDB restores the original instructions. This
behaviour guards against leaving breakpoints inserted in the
target should gdb abrubptly disconnect. However, with slow remote
targets, inserting and removing breakpoint can reduce the performance.
This behavior can be controlled with the following commands::

set breakpoint always-inserted off

All breakpoints, including newly added by the user, are inserted in
the target only when the target is resumed. All breakpoints are
removed from the target when it stops. This is the default mode.

set breakpoint always-inserted on

Causes all breakpoints to be inserted in the target at all times. If
the user adds a new breakpoint, or changes an existing breakpoint, the
breakpoints in the target are updated immediately. A breakpoint is
removed from the target only when breakpoint itself is deleted.

GDB handles conditional breakpoints by evaluating these conditions
when a breakpoint breaks. If the condition is true, then the process being
debugged stops, otherwise the process is resumed.

If the target supports evaluating conditions on its end, GDB may
download the breakpoint, together with its conditions, to it.

This feature can be controlled via the following commands:

set breakpoint condition-evaluation host

This option commands GDB to evaluate the breakpoint
conditions on the host’s side. Unconditional breakpoints are sent to
the target which in turn receives the triggers and reports them back to GDB
for condition evaluation. This is the standard evaluation mode.

set breakpoint condition-evaluation target

This option commands GDB to download breakpoint conditions
to the target at the moment of their insertion. The target
is responsible for evaluating the conditional expression and reporting
breakpoint stop events back to GDB whenever the condition
is true. Due to limitations of target-side evaluation, some conditions
cannot be evaluated there, e.g., conditions that depend on local data
that is only known to the host. Examples include
conditional expressions involving convenience variables, complex types
that cannot be handled by the agent expression parser and expressions
that are too long to be sent over to the target, specially when the
target is a remote system. In these cases, the conditions will be
evaluated by GDB.

set breakpoint condition-evaluation auto

This is the default mode. If the target supports evaluating breakpoint
conditions on its end, GDB will download breakpoint conditions to
the target (limitations mentioned previously apply). If the target does
not support breakpoint condition evaluation, then GDB will fallback
to evaluating all these conditions on the host’s side.

GDB itself sometimes sets breakpoints in your program for
special purposes, such as proper handling of longjmp (in C
programs). These internal breakpoints are assigned negative numbers,
starting with -1; ‘info breakpoints’ does not display them.
You can see these breakpoints with the GDB maintenance command
‘maint info breakpoints’ (see maint info breakpoints).

5.1.2 Setting Watchpoints

You can use a watchpoint to stop execution whenever the value of an
expression changes, without having to predict a particular place where
this may happen. (This is sometimes called a data breakpoint.)
The expression may be as simple as the value of a single variable, or
as complex as many variables combined by operators. Examples include:

A reference to the value of a single variable.

An address cast to an appropriate data type. For example,
‘*(int *)0x12345678’ will watch a 4-byte region at the specified
address (assuming an int occupies 4 bytes).

An arbitrarily complex expression, such as ‘a*b + c/d’. The
expression can use any operators valid in the program’s native
language (see Languages).

You can set a watchpoint on an expression even if the expression can
not be evaluated yet. For instance, you can set a watchpoint on
‘*global_ptr’ before ‘global_ptr’ is initialized.
GDB will stop when your program sets ‘global_ptr’ and
the expression produces a valid value. If the expression becomes
valid in some other way than changing a variable (e.g. if the memory
pointed to by ‘*global_ptr’ becomes readable as the result of a
malloc call), GDB may not stop until the next time
the expression changes.

Depending on your system, watchpoints may be implemented in software or
hardware. GDB does software watchpointing by single-stepping your
program and testing the variable’s value each time, which is hundreds of
times slower than normal execution. (But this may still be worth it, to
catch errors where you have no clue what part of your program is the
culprit.)

On some systems, such as most PowerPC or x86-based targets,
GDB includes support for hardware watchpoints, which do not
slow down the running of your program.

watch [-l|-location]expr[thread thread-id][mask maskvalue]

Set a watchpoint for an expression. GDB will break when the
expression expr is written into by the program and its value
changes. The simplest (and the most popular) use of this command is
to watch the value of a single variable:

(gdb) watch foo

If the command includes a [thread thread-id]
argument, GDB breaks only when the thread identified by
thread-id changes the value of expr. If any other threads
change the value of expr, GDB will not break. Note
that watchpoints restricted to a single thread in this way only work
with Hardware Watchpoints.

Ordinarily a watchpoint respects the scope of variables in expr
(see below). The -location argument tells GDB to
instead watch the memory referred to by expr. In this case,
GDB will evaluate expr, take the address of the result,
and watch the memory at that address. The type of the result is used
to determine the size of the watched memory. If the expression’s
result does not have an address, then GDB will print an
error.

The [mask maskvalue] argument allows creation
of masked watchpoints, if the current architecture supports this
feature (e.g., PowerPC Embedded architecture, see PowerPC Embedded.) A masked watchpoint specifies a mask in addition
to an address to watch. The mask specifies that some bits of an address
(the bits which are reset in the mask) should be ignored when matching
the address accessed by the inferior against the watchpoint address.
Thus, a masked watchpoint watches many addresses simultaneously—those
addresses whose unmasked bits are identical to the unmasked bits in the
watchpoint address. The mask argument implies -location.
Examples:

Set a watchpoint that will break when the value of expr is read
by the program.

awatch [-l|-location]expr[thread thread-id][mask maskvalue]

Set a watchpoint that will break when expr is either read from
or written into by the program.

info watchpoints [list…]

This command prints a list of watchpoints, using the same format as
info break (see Set Breaks).

If you watch for a change in a numerically entered address you need to
dereference it, as the address itself is just a constant number which will
never change. GDB refuses to create a watchpoint that watches
a never-changing value:

GDB sets a hardware watchpoint if possible. Hardware
watchpoints execute very quickly, and the debugger reports a change in
value at the exact instruction where the change occurs. If GDB
cannot set a hardware watchpoint, it sets a software watchpoint, which
executes more slowly and reports the change in value at the next
statement, not the instruction, after the change occurs.

You can force GDB to use only software watchpoints with the
set can-use-hw-watchpoints 0 command. With this variable set to
zero, GDB will never try to use hardware watchpoints, even if
the underlying system supports them. (Note that hardware-assisted
watchpoints that were set before setting
can-use-hw-watchpoints to zero will still use the hardware
mechanism of watching expression values.)

Currently, the awatch and rwatch commands can only set
hardware watchpoints, because accesses to data that don’t change the
value of the watched expression cannot be detected without examining
every instruction as it is being executed, and GDB does not do
that currently. If GDB finds that it is unable to set a
hardware breakpoint with the awatch or rwatch command, it
will print a message like this:

Expression cannot be implemented with read/access watchpoint.

Sometimes, GDB cannot set a hardware watchpoint because the
data type of the watched expression is wider than what a hardware
watchpoint on the target machine can handle. For example, some systems
can only watch regions that are up to 4 bytes wide; on such systems you
cannot set hardware watchpoints for an expression that yields a
double-precision floating-point number (which is typically 8 bytes
wide). As a work-around, it might be possible to break the large region
into a series of smaller ones and watch them with separate watchpoints.

If you set too many hardware watchpoints, GDB might be unable
to insert all of them when you resume the execution of your program.
Since the precise number of active watchpoints is unknown until such
time as the program is about to be resumed, GDB might not be
able to warn you about this when you set the watchpoints, and the
warning will be printed only when the program is resumed:

Hardware watchpoint num: Could not insert watchpoint

If this happens, delete or disable some of the watchpoints.

Watching complex expressions that reference many variables can also
exhaust the resources available for hardware-assisted watchpoints.
That’s because GDB needs to watch every variable in the
expression with separately allocated resources.

If you call a function interactively using print or call,
any watchpoints you have set will be inactive until GDB reaches another
kind of breakpoint or the call completes.

GDB automatically deletes watchpoints that watch local
(automatic) variables, or expressions that involve such variables, when
they go out of scope, that is, when the execution leaves the block in
which these variables were defined. In particular, when the program
being debugged terminates, all local variables go out of scope,
and so only watchpoints that watch global variables remain set. If you
rerun the program, you will need to set all such watchpoints again. One
way of doing that would be to set a code breakpoint at the entry to the
main function and when it breaks, set all the watchpoints.

In multi-threaded programs, watchpoints will detect changes to the
watched expression from every thread.

Warning: In multi-threaded programs, software watchpoints
have only limited usefulness. If GDB creates a software
watchpoint, it can only watch the value of an expression in a
single thread. If you are confident that the expression can only
change due to the current thread’s activity (and if you are also
confident that no other thread can become current), then you can use
software watchpoints as usual. However, GDB may not notice
when a non-current thread’s activity changes the expression. (Hardware
watchpoints, in contrast, watch an expression in all threads.)

5.1.3 Setting Catchpoints

You can use catchpoints to cause the debugger to stop for certain
kinds of program events, such as C++ exceptions or the loading of a
shared library. Use the catch command to set a catchpoint.

catch event

Stop when event occurs. The event can be any of the following:

throw [regexp]

rethrow [regexp]

catch [regexp]

The throwing, re-throwing, or catching of a C++ exception.

If regexp is given, then only exceptions whose type matches the
regular expression will be caught.

The convenience variable $_exception is available at an
exception-related catchpoint, on some systems. This holds the
exception being thrown.

There are currently some limitations to C++ exception handling in
GDB:

The support for these commands is system-dependent. Currently, only
systems using the ‘gnu-v3’ C++ ABI (see ABI) are
supported.

The regular expression feature and the $_exception convenience
variable rely on the presence of some SDT probes in libstdc++.
If these probes are not present, then these features cannot be used.
These probes were first available in the GCC 4.8 release, but whether
or not they are available in your GCC also depends on how it was
built.

The $_exception convenience variable is only valid at the
instruction at which an exception-related catchpoint is set.

When an exception-related catchpoint is hit, GDB stops at a
location in the system library which implements runtime exception
support for C++, usually libstdc++. You can use up
(see Selection) to get to your code.

If you call a function interactively, GDB normally returns
control to you when the function has finished executing. If the call
raises an exception, however, the call may bypass the mechanism that
returns control to you and cause your program either to abort or to
simply continue running until it hits a breakpoint, catches a signal
that GDB is listening for, or exits. This is the case even if
you set a catchpoint for the exception; catchpoints on exceptions are
disabled within interactive calls. See Calling, for information on
controlling this with set unwind-on-terminating-exception.

You cannot raise an exception interactively.

You cannot install an exception handler interactively.

exception

An Ada exception being raised. If an exception name is specified
at the end of the command (eg catch exception Program_Error),
the debugger will stop only when this specific exception is raised.
Otherwise, the debugger stops execution when any Ada exception is raised.

When inserting an exception catchpoint on a user-defined exception whose
name is identical to one of the exceptions defined by the language, the
fully qualified name must be used as the exception name. Otherwise,
GDB will assume that it should stop on the pre-defined exception
rather than the user-defined one. For instance, assuming an exception
called Constraint_Error is defined in package Pck, then
the command to use to catch such exceptions is catch exception
Pck.Constraint_Error.

handlers

An Ada exception being handled. If an exception name is
specified at the end of the command
(eg catch handlers Program_Error), the debugger will stop
only when this specific exception is handled.
Otherwise, the debugger stops execution when any Ada exception is handled.

When inserting a handlers catchpoint on a user-defined
exception whose name is identical to one of the exceptions
defined by the language, the fully qualified name must be used
as the exception name. Otherwise, GDB will assume that it
should stop on the pre-defined exception rather than the
user-defined one. For instance, assuming an exception called
Constraint_Error is defined in package Pck, then the
command to use to catch such exceptions handling is
catch handlers Pck.Constraint_Error.

exception unhandled

An exception that was raised but is not handled by the program.

assert

A failed Ada assertion.

exec

A call to exec.

syscall

syscall [name|number|group:groupname|g:groupname] …

A call to or return from a system call, a.k.a. syscall. A
syscall is a mechanism for application programs to request a service
from the operating system (OS) or one of the OS system services.
GDB can catch some or all of the syscalls issued by the
debuggee, and show the related information for each syscall. If no
argument is specified, calls to and returns from all system calls
will be caught.

name can be any system call name that is valid for the
underlying OS. Just what syscalls are valid depends on the OS. On
GNU and Unix systems, you can find the full list of valid syscall
names on /usr/include/asm/unistd.h.

Normally, GDB knows in advance which syscalls are valid for
each OS, so you can use the GDB command-line completion
facilities (see command completion) to list the
available choices.

You may also specify the system call numerically. A syscall’s
number is the value passed to the OS’s syscall dispatcher to
identify the requested service. When you specify the syscall by its
name, GDB uses its database of syscalls to convert the name
into the corresponding numeric code, but using the number directly
may be useful if GDB’s database does not have the complete
list of syscalls on your system (e.g., because GDB lags
behind the OS upgrades).

You may specify a group of related syscalls to be caught at once using
the group: syntax (g: is a shorter equivalent). For
instance, on some platforms GDB allows you to catch all
network related syscalls, by passing the argument group:network
to catch syscall. Note that not all syscall groups are
available in every system. You can use the command completion
facilities (see command completion) to list the
syscall groups available on your environment.

The example below illustrates how this command works if you don’t provide
arguments to it:

However, there can be situations when there is no corresponding name
in XML file for that syscall number. In this case, GDB prints
a warning message saying that it was not able to find the syscall name,
but the catchpoint will be set anyway. See the example below:

(gdb) catch syscall 764
warning: The number '764' does not represent a known syscall.
Catchpoint 2 (syscall 764)
(gdb)

If you configure GDB using the ‘--without-expat’ option,
it will not be able to display syscall names. Also, if your
architecture does not have an XML file describing its system calls,
you will not be able to see the syscall names. It is important to
notice that these two features are used for accessing the syscall
name database. In either case, you will see a warning like this:

(gdb) catch syscall
warning: Could not open "syscalls/i386-linux.xml"
warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'.
GDB will not be able to display syscall names.
Catchpoint 1 (syscall)
(gdb)

Of course, the file name will change depending on your architecture and system.

Still using the example above, you can also try to catch a syscall by its
number. In this case, you would see something like:

(gdb) catch syscall 252
Catchpoint 1 (syscall(s) 252)

Again, in this case GDB would not be able to display syscall’s names.

fork

A call to fork.

vfork

A call to vfork.

load [regexp]

unload [regexp]

The loading or unloading of a shared library. If regexp is
given, then the catchpoint will stop only if the regular expression
matches one of the affected libraries.

signal [signal… | ‘all’]

The delivery of a signal.

With no arguments, this catchpoint will catch any signal that is not
used internally by GDB, specifically, all signals except
‘SIGTRAP’ and ‘SIGINT’.

With the argument ‘all’, all signals, including those used by
GDB, will be caught. This argument cannot be used with other
signal names.

Otherwise, the arguments are a list of signal names as given to
handle (see Signals). Only signals specified in this list
will be caught.

One reason that catch signal can be more useful than
handle is that you can attach commands and conditions to the
catchpoint.

When a signal is caught by a catchpoint, the signal’s stop and
print settings, as specified by handle, are ignored.
However, whether the signal is still delivered to the inferior depends
on the pass setting; this can be changed in the catchpoint’s
commands.

tcatch event

Set a catchpoint that is enabled only for one stop. The catchpoint is
automatically deleted after the first time the event is caught.

5.1.4 Deleting Breakpoints

It is often necessary to eliminate a breakpoint, watchpoint, or
catchpoint once it has done its job and you no longer want your program
to stop there. This is called deleting the breakpoint. A
breakpoint that has been deleted no longer exists; it is forgotten.

With the clear command you can delete breakpoints according to
where they are in your program. With the delete command you can
delete individual breakpoints, watchpoints, or catchpoints by specifying
their breakpoint numbers.

It is not necessary to delete a breakpoint to proceed past it. GDB
automatically ignores breakpoints on the first instruction to be executed
when you continue execution without changing the execution address.

clear

Delete any breakpoints at the next instruction to be executed in the
selected stack frame (see Selecting a Frame). When
the innermost frame is selected, this is a good way to delete a
breakpoint where your program just stopped.

clear location

Delete any breakpoints set at the specified location.
See Specify Location, for the various forms of location; the
most useful ones are listed below:

clear function

clear filename:function

Delete any breakpoints set at entry to the named function.

clear linenum

clear filename:linenum

Delete any breakpoints set at or within the code of the specified
linenum of the specified filename.

delete [breakpoints][list…]

Delete the breakpoints, watchpoints, or catchpoints of the breakpoint
list specified as argument. If no argument is specified, delete all
breakpoints (GDB asks confirmation, unless you have set
confirm off). You can abbreviate this command as d.

5.1.5 Disabling Breakpoints

Rather than deleting a breakpoint, watchpoint, or catchpoint, you might
prefer to disable it. This makes the breakpoint inoperative as if
it had been deleted, but remembers the information on the breakpoint so
that you can enable it again later.

You disable and enable breakpoints, watchpoints, and catchpoints with
the enable and disable commands, optionally specifying
one or more breakpoint numbers as arguments. Use info break to
print a list of all breakpoints, watchpoints, and catchpoints if you
do not know which numbers to use.

Disabling and enabling a breakpoint that has multiple locations
affects all of its locations.

A breakpoint, watchpoint, or catchpoint can have any of several
different states of enablement:

Enabled. The breakpoint stops your program. A breakpoint set
with the break command starts out in this state.

Enabled for a count. The breakpoint stops your program for the next
N times, then becomes disabled.

Enabled for deletion. The breakpoint stops your program, but
immediately after it does so it is deleted permanently. A breakpoint
set with the tbreak command starts out in this state.

You can use the following commands to enable or disable breakpoints,
watchpoints, and catchpoints:

disable [breakpoints][list…]

Disable the specified breakpoints—or all breakpoints, if none are
listed. A disabled breakpoint has no effect but is not forgotten. All
options such as ignore-counts, conditions and commands are remembered in
case the breakpoint is enabled again later. You may abbreviate
disable as dis.

enable [breakpoints][list…]

Enable the specified breakpoints (or all defined breakpoints). They
become effective once again in stopping your program.

enable [breakpoints] once list…

Enable the specified breakpoints temporarily. GDB disables any
of these breakpoints immediately after stopping your program.

enable [breakpoints] count countlist…

Enable the specified breakpoints temporarily. GDB records
count with each of the specified breakpoints, and decrements a
breakpoint’s count when it is hit. When any count reaches 0,
GDB disables that breakpoint. If a breakpoint has an ignore
count (see Break Conditions), that will be
decremented to 0 before count is affected.

enable [breakpoints] delete list…

Enable the specified breakpoints to work once, then die. GDB
deletes any of these breakpoints as soon as your program stops there.
Breakpoints set by the tbreak command start out in this state.

Except for a breakpoint set with tbreak (see Setting Breakpoints), breakpoints that you set are initially enabled;
subsequently, they become disabled or enabled only when you use one of
the commands above. (The command until can set and delete a
breakpoint of its own, but it does not change the state of your other
breakpoints; see Continuing and
Stepping.)

5.1.6 Break Conditions

The simplest sort of breakpoint breaks every time your program reaches a
specified place. You can also specify a condition for a
breakpoint. A condition is just a Boolean expression in your
programming language (see Expressions). A breakpoint with
a condition evaluates the expression each time your program reaches it,
and your program stops only if the condition is true.

This is the converse of using assertions for program validation; in that
situation, you want to stop when the assertion is violated—that is,
when the condition is false. In C, if you want to test an assertion expressed
by the condition assert, you should set the condition
‘! assert’ on the appropriate breakpoint.

Conditions are also accepted for watchpoints; you may not need them,
since a watchpoint is inspecting the value of an expression anyhow—but
it might be simpler, say, to just set a watchpoint on a variable name,
and specify a condition that tests whether the new value is an interesting
one.

Break conditions can have side effects, and may even call functions in
your program. This can be useful, for example, to activate functions
that log program progress, or to use your own print functions to
format special data structures. The effects are completely predictable
unless there is another enabled breakpoint at the same address. (In
that case, GDB might see the other breakpoint first and stop your
program without checking the condition of this one.) Note that
breakpoint commands are usually more convenient and flexible than break
conditions for the
purpose of performing side effects when a breakpoint is reached
(see Breakpoint Command Lists).

Breakpoint conditions can also be evaluated on the target’s side if
the target supports it. Instead of evaluating the conditions locally,
GDB encodes the expression into an agent expression
(see Agent Expressions) suitable for execution on the target,
independently of GDB. Global variables become raw memory
locations, locals become stack accesses, and so forth.

In this case, GDB will only be notified of a breakpoint trigger
when its condition evaluates to true. This mechanism may provide faster
response times depending on the performance characteristics of the target
since it does not need to keep GDB informed about
every breakpoint trigger, even those with false conditions.

Break conditions can be specified when a breakpoint is set, by using
‘if’ in the arguments to the break command. See Setting Breakpoints. They can also be changed at any time
with the condition command.

You can also use the if keyword with the watch command.
The catch command does not recognize the if keyword;
condition is the only way to impose a further condition on a
catchpoint.

condition bnumexpression

Specify expression as the break condition for breakpoint,
watchpoint, or catchpoint number bnum. After you set a condition,
breakpoint bnum stops your program only if the value of
expression is true (nonzero, in C). When you use
condition, GDB checks expression immediately for
syntactic correctness, and to determine whether symbols in it have
referents in the context of your breakpoint. If expression uses
symbols not referenced in the context of the breakpoint, GDB
prints an error message:

No symbol "foo" in current context.

GDB does
not actually evaluate expression at the time the condition
command (or a command that sets a breakpoint with a condition, like
break if …) is given, however. See Expressions.

condition bnum

Remove the condition from breakpoint number bnum. It becomes
an ordinary unconditional breakpoint.

A special case of a breakpoint condition is to stop only when the
breakpoint has been reached a certain number of times. This is so
useful that there is a special way to do it, using the ignore
count of the breakpoint. Every breakpoint has an ignore count, which
is an integer. Most of the time, the ignore count is zero, and
therefore has no effect. But if your program reaches a breakpoint whose
ignore count is positive, then instead of stopping, it just decrements
the ignore count by one and continues. As a result, if the ignore count
value is n, the breakpoint does not stop the next n times
your program reaches it.

ignore bnumcount

Set the ignore count of breakpoint number bnum to count.
The next count times the breakpoint is reached, your program’s
execution does not stop; other than to decrement the ignore count, GDB
takes no action.

To make the breakpoint stop the next time it is reached, specify
a count of zero.

When you use continue to resume execution of your program from a
breakpoint, you can specify an ignore count directly as an argument to
continue, rather than using ignore. See Continuing and Stepping.

If a breakpoint has a positive ignore count and a condition, the
condition is not checked. Once the ignore count reaches zero,
GDB resumes checking the condition.

You could achieve the effect of the ignore count with a condition such
as ‘$foo-- <= 0’ using a debugger convenience variable that
is decremented each time. See Convenience
Variables.

5.1.7 Breakpoint Command Lists

You can give any breakpoint (or watchpoint or catchpoint) a series of
commands to execute when your program stops due to that breakpoint. For
example, you might want to print the values of certain expressions, or
enable other breakpoints.

commands [list…]

… command-list …

end

Specify a list of commands for the given breakpoints. The commands
themselves appear on the following lines. Type a line containing just
end to terminate the commands.

To remove all commands from a breakpoint, type commands and
follow it immediately with end; that is, give no commands.

With no argument, commands refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered). If the most recent breakpoints were set with a single
command, then the commands will apply to all the breakpoints
set by that command. This applies to breakpoints set by
rbreak, and also applies when a single break command
creates multiple breakpoints (see Ambiguous
Expressions).

Pressing RET as a means of repeating the last GDB command is
disabled within a command-list.

You can use breakpoint commands to start your program up again. Simply
use the continue command, or step, or any other command
that resumes execution.

Any other commands in the command list, after a command that resumes
execution, are ignored. This is because any time you resume execution
(even with a simple next or step), you may encounter
another breakpoint—which could have its own command list, leading to
ambiguities about which list to execute.

If the first command you specify in a command list is silent, the
usual message about stopping at a breakpoint is not printed. This may
be desirable for breakpoints that are to print a specific message and
then continue. If none of the remaining commands print anything, you
see no sign that the breakpoint was reached. silent is
meaningful only at the beginning of a breakpoint command list.

The commands echo, output, and printf allow you to
print precisely controlled output, and are often useful in silent
breakpoints. See Commands for Controlled Output.

For example, here is how you could use breakpoint commands to print the
value of x at entry to foo whenever x is positive.

break foo if x>0
commands
silent
printf "x is %d\n",x
cont
end

One application for breakpoint commands is to compensate for one bug so
you can test for another. Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them. End with the continue command
so that your program does not stop, and start with the silent
command so that no output is produced. Here is an example:

5.1.8 Dynamic Printf

The dynamic printf command dprintf combines a breakpoint with
formatted printing of your program’s data to give you the effect of
inserting printf calls into your program on-the-fly, without
having to recompile it.

In its most basic form, the output goes to the GDB console. However,
you can set the variable dprintf-style for alternate handling.
For instance, you can ask to format the output by calling your
program’s printf function. This has the advantage that the
characters go to the program’s output device, so they can recorded in
redirects to files and so forth.

If you are doing remote debugging with a stub or agent, you can also
ask to have the printf handled by the remote agent. In addition to
ensuring that the output goes to the remote program’s device along
with any other output the program might produce, you can also ask that
the dprintf remain active even after disconnecting from the remote
target. Using the stub/agent is also more efficient, as it can do
everything without needing to communicate with GDB.

dprintf location,template,expression[,expression…]

Whenever execution reaches location, print the values of one or
more expressions under the control of the string template.
To print several values, separate them with commas.

set dprintf-style style

Set the dprintf output to be handled in one of several different
styles enumerated below. A change of style affects all existing
dynamic printfs immediately. (If you need individual control over the
print commands, simply define normal breakpoints with
explicitly-supplied command lists.)

gdb

Handle the output using the GDBprintf command.

call

Handle the output by calling a function in your program (normally
printf).

agent

Have the remote debugging agent (such as gdbserver) handle
the output itself. This style is only available for agents that
support running commands on the target.

set dprintf-function function

Set the function to call if the dprintf style is call. By
default its value is printf. You may set it to any expression.
that GDB can evaluate to a function, as per the call
command.

set dprintf-channel channel

Set a “channel” for dprintf. If set to a non-empty value,
GDB will evaluate it as an expression and pass the result as
a first argument to the dprintf-function, in the manner of
fprintf and similar functions. Otherwise, the dprintf format
string will be the first argument, in the manner of printf.

As an example, if you wanted dprintf output to go to a logfile
that is a standard I/O stream assigned to the variable mylog,
you could do the following:

Note that the info break displays the dynamic printf commands
as normal breakpoint commands; you can thus easily see the effect of
the variable settings.

set disconnected-dprintf on

set disconnected-dprintf off

Choose whether dprintf commands should continue to run if
GDB has disconnected from the target. This only applies
if the dprintf-style is agent.

show disconnected-dprintf off

Show the current choice for disconnected dprintf.

GDB does not check the validity of function and channel,
relying on you to supply values that are meaningful for the contexts
in which they are being used. For instance, the function and channel
may be the values of local variables, but if that is the case, then
all enabled dynamic prints must be at locations within the scope of
those locals. If evaluation fails, GDB will report an error.

5.1.9 How to save breakpoints to a file

To save breakpoint definitions to a file use the save breakpoints command.

save breakpoints [filename]

This command saves all current breakpoint definitions together with
their commands and ignore counts, into a file filename
suitable for use in a later debugging session. This includes all
types of breakpoints (breakpoints, watchpoints, catchpoints,
tracepoints). To read the saved breakpoint definitions, use the
source command (see Command Files). Note that watchpoints
with expressions involving local variables may fail to be recreated
because it may not be possible to access the context where the
watchpoint is valid anymore. Because the saved breakpoint definitions
are simply a sequence of GDB commands that recreate the
breakpoints, you can edit the file in your favorite editing program,
and remove the breakpoint definitions you’re not interested in, or
that can no longer be recreated.

Some SystemTap probes have an associated semaphore variable;
for instance, this happens automatically if you defined your probe
using a DTrace-style .d file. If your probe has a semaphore,
GDB will automatically enable it when you specify a
breakpoint using the ‘-probe-stap’ notation. But, if you put a
breakpoint at a probe’s location by some other method (e.g.,
break file:line), then GDB will not automatically set
the semaphore. DTrace probes do not support semaphores.

You can examine the available static static probes using info
probes, with optional arguments:

info probes [type][provider[name[objfile]]]

If given, type is either stap for listing
SystemTap probes or dtrace for listing DTrace
probes. If omitted all probes are listed regardless of their types.

If given, provider is a regular expression used to match against provider
names when selecting which probes to list. If omitted, probes by all
probes from all providers are listed.

If given, name is a regular expression to match against probe names
when selecting which probes to list. If omitted, probe names are not
considered when deciding whether to display them.

If given, objfile is a regular expression used to select which
object files (executable or shared libraries) to examine. If not
given, all object files are considered.

info probes all

List the available static probes, from all types.

Some probe points can be enabled and/or disabled. The effect of
enabling or disabling a probe depends on the type of probe being
handled. Some DTrace probes can be enabled or
disabled, but SystemTap probes cannot be disabled.

You can enable (or disable) one or more probes using the following
commands, with optional arguments:

enable probes [provider[name[objfile]]]

If given, provider is a regular expression used to match against
provider names when selecting which probes to enable. If omitted,
all probes from all providers are enabled.

If given, name is a regular expression to match against probe
names when selecting which probes to enable. If omitted, probe names
are not considered when deciding whether to enable them.

If given, objfile is a regular expression used to select which
object files (executable or shared libraries) to examine. If not
given, all object files are considered.

disable probes [provider[name[objfile]]]

See the enable probes command above for a description of the
optional arguments accepted by this command.

A probe may specify up to twelve arguments. These are available at the
point at which the probe is defined—that is, when the current PC is
at the probe’s location. The arguments are available using the
convenience variables (see Convenience Vars)
$_probe_arg0…$_probe_arg11. In SystemTap
probes each probe argument is an integer of the appropriate size;
types are not preserved. In DTrace probes types are preserved
provided that they are recognized as such by GDB; otherwise
the value of the probe argument will be a long integer. The
convenience variable $_probe_argc holds the number of arguments
at the current probe point.

These variables are always available, but attempts to access them at
any location other than a probe point will cause GDB to give
an error message.

5.1.12 “Breakpoint address adjusted...”

Some processor architectures place constraints on the addresses at
which breakpoints may be placed. For architectures thus constrained,
GDB will attempt to adjust the breakpoint’s address to comply
with the constraints dictated by the architecture.

One example of such an architecture is the Fujitsu FR-V. The FR-V is
a VLIW architecture in which a number of RISC-like instructions may be
bundled together for parallel execution. The FR-V architecture
constrains the location of a breakpoint instruction within such a
bundle to the instruction with the lowest address. GDB
honors this constraint by adjusting a breakpoint’s address to the
first in the bundle.

It is not uncommon for optimized code to have bundles which contain
instructions from different source statements, thus it may happen that
a breakpoint’s address will be adjusted from one source statement to
another. Since this adjustment may significantly alter GDB’s
breakpoint related behavior from what the user expects, a warning is
printed when the breakpoint is first set and also when the breakpoint
is hit.

A warning like the one below is printed when setting a breakpoint
that’s been subject to address adjustment:

warning: Breakpoint address adjusted from 0x00010414 to 0x00010410.

Such warnings are printed both for user settable and GDB’s
internal breakpoints. If you see one of these warnings, you should
verify that a breakpoint set at the adjusted address will have the
desired affect. If not, the breakpoint in question may be removed and
other breakpoints may be set which will have the desired behavior.
E.g., it may be sufficient to place the breakpoint at a later
instruction. A conditional breakpoint may also be useful in some
cases to prevent the breakpoint from triggering too often.

GDB will also issue a warning when stopping at one of these
adjusted breakpoints:

5.2 Continuing and Stepping

Continuing means resuming program execution until your program
completes normally. In contrast, stepping means executing just
one more “step” of your program, where “step” may mean either one
line of source code, or one machine instruction (depending on what
particular command you use). Either when continuing or when stepping,
your program may stop even sooner, due to a breakpoint or a signal. (If
it stops due to a signal, you may want to use handle, or use
‘signal 0’ to resume execution (see Signals),
or you may step into the signal’s handler (see stepping and signal handlers).)

continue [ignore-count]

c [ignore-count]

fg [ignore-count]

Resume program execution, at the address where your program last stopped;
any breakpoints set at that address are bypassed. The optional argument
ignore-count allows you to specify a further number of times to
ignore a breakpoint at this location; its effect is like that of
ignore (see Break Conditions).

The argument ignore-count is meaningful only when your program
stopped due to a breakpoint. At other times, the argument to
continue is ignored.

The synonyms c and fg (for foreground, as the
debugged program is deemed to be the foreground program) are provided
purely for convenience, and have exactly the same behavior as
continue.

A typical technique for using stepping is to set a breakpoint
(see Breakpoints; Watchpoints; and Catchpoints) at the
beginning of the function or the section of your program where a problem
is believed to lie, run your program until it stops at that breakpoint,
and then step through the suspect area, examining the variables that are
interesting, until you see the problem happen.

step

Continue running your program until control reaches a different source
line, then stop it and return control to GDB. This command is
abbreviated s.

Warning: If you use the step command while control is
within a function that was compiled without debugging information,
execution proceeds until control reaches a function that does have
debugging information. Likewise, it will not step into a function which
is compiled without debugging information. To step through functions
without debugging information, use the stepi command, described
below.

The step command only stops at the first instruction of a source
line. This prevents the multiple stops that could otherwise occur in
switch statements, for loops, etc. step continues
to stop if a function that has debugging information is called within
the line. In other words, stepsteps inside any functions
called within the line.

Also, the step command only enters a function if there is line
number information for the function. Otherwise it acts like the
next command. This avoids problems when using cc -gl
on MIPS machines. Previously, step entered subroutines if there
was any debugging information about the routine.

step count

Continue running as in step, but do so count times. If a
breakpoint is reached, or a signal not related to stepping occurs before
count steps, stepping stops right away.

next [count]

Continue to the next source line in the current (innermost) stack frame.
This is similar to step, but function calls that appear within
the line of code are executed without stopping. Execution stops when
control reaches a different line of code at the original stack level
that was executing when you gave the next command. This command
is abbreviated n.

An argument count is a repeat count, as for step.

The next command only stops at the first instruction of a
source line. This prevents multiple stops that could otherwise occur in
switch statements, for loops, etc.

set step-mode

set step-mode on

The set step-mode on command causes the step command to
stop at the first instruction of a function which contains no debug line
information rather than stepping over it.

This is useful in cases where you may be interested in inspecting the
machine instructions of a function which has no symbolic info and do not
want GDB to automatically skip over this function.

set step-mode off

Causes the step command to step over any functions which contains no
debug information. This is the default.

show step-mode

Show whether GDB will stop in or step over functions without
source line debug information.

finish

Continue running until just after function in the selected stack frame
returns. Print the returned value (if any). This command can be
abbreviated as fin.

Continue running until a source line past the current line, in the
current stack frame, is reached. This command is used to avoid single
stepping through a loop more than once. It is like the next
command, except that when until encounters a jump, it
automatically continues execution until the program counter is greater
than the address of the jump.

This means that when you reach the end of a loop after single stepping
though it, until makes your program continue execution until it
exits the loop. In contrast, a next command at the end of a loop
simply steps back to the beginning of the loop, which forces you to step
through the next iteration.

until always stops your program if it attempts to exit the current
stack frame.

until may produce somewhat counterintuitive results if the order
of machine code does not match the order of the source lines. For
example, in the following excerpt from a debugging session, the f
(frame) command shows that execution is stopped at line
206; yet when we use until, we get to line 195:

This happened because, for execution efficiency, the compiler had
generated code for the loop closure test at the end, rather than the
start, of the loop—even though the test in a C for-loop is
written before the body of the loop. The until command appeared
to step back to the beginning of the loop when it advanced to this
expression; however, it has not really gone to an earlier
statement—not in terms of the actual machine code.

until with no argument works by means of single
instruction stepping, and hence is slower than until with an
argument.

until location

u location

Continue running your program until either the specified location is
reached, or the current stack frame returns. The location is any of
the forms described in Specify Location.
This form of the command uses temporary breakpoints, and
hence is quicker than until without an argument. The specified
location is actually reached only if it is in the current frame. This
implies that until can be used to skip over recursive function
invocations. For instance in the code below, if the current location is
line 96, issuing until 99 will execute the program up to
line 99 in the same invocation of factorial, i.e., after the inner
invocations have returned.

Continue running the program up to the given location. An argument is
required, which should be of one of the forms described in
Specify Location.
Execution will also stop upon exit from the current stack
frame. This command is similar to until, but advance will
not skip over recursive function calls, and the target location doesn’t
have to be in the same frame as the current one.

stepi

stepi arg

si

Execute one machine instruction, then stop and return to the debugger.

It is often useful to do ‘display/i $pc’ when stepping by machine
instructions. This makes GDB automatically display the next
instruction to be executed, each time your program stops. See Automatic Display.

An argument is a repeat count, as in step.

nexti

nexti arg

ni

Execute one machine instruction, but if it is a function call,
proceed until the function returns.

An argument is a repeat count, as in next.

By default, and if available, GDB makes use of
target-assisted range stepping. In other words, whenever you
use a stepping command (e.g., step, next), GDB
tells the target to step the corresponding range of instruction
addresses instead of issuing multiple single-steps. This speeds up
line stepping, particularly for remote targets. Ideally, there should
be no reason you would want to turn range stepping off. However, it’s
possible that a bug in the debug info, a bug in the remote stub (for
remote targets), or even a bug in GDB could make line
stepping behave incorrectly when target-assisted range stepping is
enabled. You can use the following command to turn off range stepping
if necessary:

set range-stepping

show range-stepping

Control whether range stepping is enabled.

If on, and the target supports it, GDB tells the
target to step a range of addresses itself, instead of issuing
multiple single-steps. If off, GDB always issues
single-steps, even if range stepping is supported by the target. The
default is on.

5.3 Skipping Over Functions and Files

The program you are debugging may contain some functions which are
uninteresting to debug. The skip command lets you tell GDB to
skip a function, all functions in a file or a particular function in
a particular file when stepping.

For example, consider the following C function:

101 int func()
102 {
103 foo(boring());
104 bar(boring());
105 }

Suppose you wish to step into the functions foo and bar, but you
are not interested in stepping through boring. If you run step
at line 103, you’ll enter boring(), but if you run next, you’ll
step over both foo and boring!

One solution is to step into boring and use the finish
command to immediately exit it. But this can become tedious if boring
is called from many places.

A more flexible solution is to execute skip boring. This instructs
GDB never to step into boring. Now when you execute
step at line 103, you’ll step over boring and directly into
foo.

Functions may be skipped by providing either a function name, linespec
(see Specify Location), regular expression that matches the function’s
name, file name or a glob-style pattern that matches the file name.

On Posix systems the form of the regular expression is
“Extended Regular Expressions”. See for example ‘man 7 regex’
on GNU/Linux systems. On non-Posix systems the form of the regular
expression is whatever is provided by the regcomp function of
the underlying system.
See for example ‘man 7 glob’ on GNU/Linux systems for a
description of glob-style patterns.

skip [options]

The basic form of the skip command takes zero or more options
that specify what to skip.
The options argument is any useful combination of the following:

-file file

-fi file

Functions in file will be skipped over when stepping.

-gfile file-glob-pattern

-gfi file-glob-pattern

Functions in files matching file-glob-pattern will be skipped
over when stepping.

(gdb) skip -gfi utils/*.c

-function linespec

-fu linespec

Functions named by linespec or the function containing the line
named by linespec will be skipped over when stepping.
See Specify Location.

-rfunction regexp

-rfu regexp

Functions whose name matches regexp will be skipped over when stepping.

This form is useful for complex function names.
For example, there is generally no need to step into C++std::string
constructors or destructors. Plus with C++ templates it can be hard to
write out the full name of the function, and often it doesn’t matter what
the template arguments are. Specifying the function to be skipped as a
regular expression makes this easier.

(gdb) skip -rfu ^std::(allocator|basic_string)<.*>::~?\1 *\(

If you want to skip every templated C++ constructor and destructor
in the std namespace you can do:

(gdb) skip -rfu ^std::([a-zA-z0-9_]+)<.*>::~?\1 *\(

If no options are specified, the function you’re currently debugging
will be skipped.

skip function [linespec]

After running this command, the function named by linespec or the
function containing the line named by linespec will be skipped over when
stepping. See Specify Location.

If you do not specify linespec, the function you’re currently debugging
will be skipped.

(If you have a function called file that you want to skip, use
skip function file.)

skip file [filename]

After running this command, any function whose source lives in filename
will be skipped over when stepping.

(gdb) skip file boring.c
File boring.c will be skipped when stepping.

If you do not specify filename, functions whose source lives in the file
you’re currently debugging will be skipped.

Skips can be listed, deleted, disabled, and enabled, much like breakpoints.
These are the commands for managing your list of skips:

info skip [range]

Print details about the specified skip(s). If range is not specified,
print a table with details about all functions and files marked for skipping.
info skip prints the following information about each skip:

Identifier

A number identifying this skip.

Enabled or Disabled

Enabled skips are marked with ‘y’.
Disabled skips are marked with ‘n’.

Glob

If the file name is a ‘glob’ pattern this is ‘y’.
Otherwise it is ‘n’.

File

The name or ‘glob’ pattern of the file to be skipped.
If no file is specified this is ‘<none>’.

RE

If the function name is a ‘regular expression’ this is ‘y’.
Otherwise it is ‘n’.

Function

The name or regular expression of the function to skip.
If no function is specified this is ‘<none>’.

skip delete [range]

Delete the specified skip(s). If range is not specified, delete all
skips.

skip enable [range]

Enable the specified skip(s). If range is not specified, enable all
skips.

skip disable [range]

Disable the specified skip(s). If range is not specified, disable all
skips.

5.4 Signals

A signal is an asynchronous event that can happen in a program. The
operating system defines the possible kinds of signals, and gives each
kind a name and a number. For example, in Unix SIGINT is the
signal a program gets when you type an interrupt character (often Ctrl-c);
SIGSEGV is the signal a program gets from referencing a place in
memory far away from all the areas in use; SIGALRM occurs when
the alarm clock timer goes off (which happens only if your program has
requested an alarm).

Some signals, including SIGALRM, are a normal part of the
functioning of your program. Others, such as SIGSEGV, indicate
errors; these signals are fatal (they kill your program immediately) if the
program has not specified in advance some other way to handle the signal.
SIGINT does not indicate an error in your program, but it is normally
fatal so it can carry out the purpose of the interrupt: to kill the program.

GDB has the ability to detect any occurrence of a signal in your
program. You can tell GDB in advance what to do for each kind of
signal.

Normally, GDB is set up to let the non-erroneous signals like
SIGALRM be silently passed to your program
(so as not to interfere with their role in the program’s functioning)
but to stop your program immediately whenever an error signal happens.
You can change these settings with the handle command.

info signals

info handle

Print a table of all the kinds of signals and how GDB has been told to
handle each one. You can use this to see the signal numbers of all
the defined types of signals.

info signals sig

Similar, but print information only about the specified signal number.

info handle is an alias for info signals.

catch signal [signal… | ‘all’]

Set a catchpoint for the indicated signals. See Set Catchpoints,
for details about this command.

handle signal[keywords…]

Change the way GDB handles signal signal. The signal
can be the number of a signal or its name (with or without the
‘SIG’ at the beginning); a list of signal numbers of the form
‘low-high’; or the word ‘all’, meaning all the
known signals. Optional arguments keywords, described below,
say what change to make.

The keywords allowed by the handle command can be abbreviated.
Their full names are:

nostop

GDB should not stop your program when this signal happens. It may
still print a message telling you that the signal has come in.

stop

GDB should stop your program when this signal happens. This implies
the print keyword as well.

print

GDB should print a message when this signal happens.

noprint

GDB should not mention the occurrence of the signal at all. This
implies the nostop keyword as well.

pass

noignore

GDB should allow your program to see this signal; your program
can handle the signal, or else it may terminate if the signal is fatal
and not handled. pass and noignore are synonyms.

nopass

ignore

GDB should not allow your program to see this signal.
nopass and ignore are synonyms.

When a signal stops your program, the signal is not visible to the
program until you
continue. Your program sees the signal then, if pass is in
effect for the signal in question at that time. In other words,
after GDB reports a signal, you can use the handle
command with pass or nopass to control whether your
program sees that signal when you continue.

The default is set to nostop, noprint, pass for
non-erroneous signals such as SIGALRM, SIGWINCH and
SIGCHLD, and to stop, print, pass for the
erroneous signals.

You can also use the signal command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time. For example, if your program stopped
due to some sort of memory reference error, you might store correct
values into the erroneous variables and continue, hoping to see more
execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal. To prevent this,
you can continue with ‘signal 0’. See Giving your
Program a Signal.

GDB optimizes for stepping the mainline code. If a signal
that has handle nostop and handle pass set arrives while
a stepping command (e.g., stepi, step, next) is
in progress, GDB lets the signal handler run and then resumes
stepping the mainline code once the signal handler returns. In other
words, GDB steps over the signal handler. This prevents
signals that you’ve specified as not interesting (with handle
nostop) from changing the focus of debugging unexpectedly. Note that
the signal handler itself may still hit a breakpoint, stop for another
signal that has handle stop in effect, or for any other event
that normally results in stopping the stepping command sooner. Also
note that GDB still informs you that the program received a
signal if handle print is set.

If you set handle pass for a signal, and your program sets up a
handler for it, then issuing a stepping command, such as step
or stepi, when your program is stopped due to the signal will
step into the signal handler (if the target supports that).

Likewise, if you use the queue-signal command to queue a signal
to be delivered to the current thread when execution of the thread
resumes (see Giving your Program a Signal), then a
stepping command will step into the signal handler.

Here’s an example, using stepi to step to the first instruction
of SIGUSR1’s handler:

On some targets, GDB can inspect extra signal information
associated with the intercepted signal, before it is actually
delivered to the program being debugged. This information is exported
by the convenience variable $_siginfo, and consists of data
that is passed by the kernel to the signal handler at the time of the
receipt of a signal. The data type of the information itself is
target dependent. You can see the data type using the ptype
$_siginfo command. On Unix systems, it typically corresponds to the
standard siginfo_t type, as defined in the signal.h
system header.

Here’s an example, on a GNU/Linux system, printing the stray
referenced address that raised a segmentation fault.

On some targets, a SIGSEGV can be caused by a boundary
violation, i.e., accessing an address outside of the allowed range.
In those cases GDB may displays additional information,
depending on how GDB has been told to handle the signal.
With handle stop SIGSEGV, GDB displays the violation
kind: "Upper" or "Lower", the memory address accessed and the
bounds, while with handle nostop SIGSEGV no additional
information is displayed.

5.5 Stopping and Starting Multi-thread Programs

GDB supports debugging programs with multiple threads
(see Debugging Programs with Multiple Threads). There
are two modes of controlling execution of your program within the
debugger. In the default mode, referred to as all-stop mode,
when any thread in your program stops (for example, at a breakpoint
or while being stepped), all other threads in the program are also stopped by
GDB. On some targets, GDB also supports
non-stop mode, in which other threads can continue to run freely while
you examine the stopped thread in the debugger.

5.5.1 All-Stop Mode

In all-stop mode, whenever your program stops under GDB for any reason,
all threads of execution stop, not just the current thread. This
allows you to examine the overall state of the program, including
switching between threads, without worrying that things may change
underfoot.

Conversely, whenever you restart the program, all threads start
executing. This is true even when single-stepping with commands
like step or next.

In particular, GDB cannot single-step all threads in lockstep.
Since thread scheduling is up to your debugging target’s operating
system (not controlled by GDB), other threads may
execute more than one statement while the current thread completes a
single step. Moreover, in general other threads stop in the middle of a
statement, rather than at a clean statement boundary, when the program
stops.

You might even find your program stopped in another thread after
continuing or even single-stepping. This happens whenever some other
thread runs into a breakpoint, a signal, or an exception before the
first thread completes whatever you requested.

Whenever GDB stops your program, due to a breakpoint or a
signal, it automatically selects the thread where that breakpoint or
signal happened. GDB alerts you to the context switch with a
message such as ‘[Switching to Thread n]’ to identify the
thread.

On some OSes, you can modify GDB’s default behavior by
locking the OS scheduler to allow only a single thread to run.

set scheduler-locking mode

Set the scheduler locking mode. It applies to normal execution,
record mode, and replay mode. If it is off, then there is no
locking and any thread may run at any time. If on, then only
the current thread may run when the inferior is resumed. The
step mode optimizes for single-stepping; it prevents other
threads from preempting the current thread while you are stepping, so
that the focus of debugging does not change unexpectedly. Other
threads never get a chance to run when you step, and they are
completely free to run when you use commands like ‘continue’,
‘until’, or ‘finish’. However, unless another thread hits a
breakpoint during its timeslice, GDB does not change the
current thread away from the thread that you are debugging. The
replay mode behaves like off in record mode and like
on in replay mode.

show scheduler-locking

Display the current scheduler locking mode.

By default, when you issue one of the execution commands such as
continue, next or step, GDB allows only
threads of the current inferior to run. For example, if GDB
is attached to two inferiors, each with two threads, the
continue command resumes only the two threads of the current
inferior. This is useful, for example, when you debug a program that
forks and you want to hold the parent stopped (so that, for instance,
it doesn’t run to exit), while you debug the child. In other
situations, you may not be interested in inspecting the current state
of any of the processes GDB is attached to, and you may want
to resume them all until some breakpoint is hit. In the latter case,
you can instruct GDB to allow all threads of all the
inferiors to run with the set schedule-multiple command.

set schedule-multiple

Set the mode for allowing threads of multiple processes to be resumed
when an execution command is issued. When on, all threads of
all processes are allowed to run. When off, only the threads
of the current process are resumed. The default is off. The
scheduler-locking mode takes precedence when set to on,
or while you are stepping and set to step.

show schedule-multiple

Display the current mode for resuming the execution of threads of
multiple processes.

5.5.2 Non-Stop Mode

For some multi-threaded targets, GDB supports an optional
mode of operation in which you can examine stopped program threads in
the debugger while other threads continue to execute freely. This
minimizes intrusion when debugging live systems, such as programs
where some threads have real-time constraints or must continue to
respond to external events. This is referred to as non-stop mode.

In non-stop mode, when a thread stops to report a debugging event,
only that thread is stopped; GDB does not stop other
threads as well, in contrast to the all-stop mode behavior. Additionally,
execution commands such as continue and step apply by default
only to the current thread in non-stop mode, rather than all threads as
in all-stop mode. This allows you to control threads explicitly in
ways that are not possible in all-stop mode — for example, stepping
one thread while allowing others to run freely, stepping
one thread while holding all others stopped, or stepping several threads
independently and simultaneously.

To enter non-stop mode, use this sequence of commands before you run
or attach to your program:

# If using the CLI, pagination breaks non-stop.
set pagination off
# Finally, turn it on!
set non-stop on

You can use these commands to manipulate the non-stop mode setting:

set non-stop on

Enable selection of non-stop mode.

set non-stop off

Disable selection of non-stop mode.

show non-stop

Show the current non-stop enablement setting.

Note these commands only reflect whether non-stop mode is enabled,
not whether the currently-executing program is being run in non-stop mode.
In particular, the set non-stop preference is only consulted when
GDB starts or connects to the target program, and it is generally
not possible to switch modes once debugging has started. Furthermore,
since not all targets support non-stop mode, even when you have enabled
non-stop mode, GDB may still fall back to all-stop operation by
default.

In non-stop mode, all execution commands apply only to the current thread
by default. That is, continue only continues one thread.
To continue all threads, issue continue -a or c -a.

You can use GDB’s background execution commands
(see Background Execution) to run some threads in the background
while you continue to examine or step others from GDB.
The MI execution commands (see GDB/MI Program Execution) are
always executed asynchronously in non-stop mode.

Suspending execution is done with the interrupt command when
running in the background, or Ctrl-c during foreground execution.
In all-stop mode, this stops the whole process;
but in non-stop mode the interrupt applies only to the current thread.
To stop the whole program, use interrupt -a.

Other execution commands do not currently support the -a option.

In non-stop mode, when a thread stops, GDB doesn’t automatically make
that thread current, as it does in all-stop mode. This is because the
thread stop notifications are asynchronous with respect to GDB’s
command interpreter, and it would be confusing if GDB unexpectedly
changed to a different thread just as you entered a command to operate on the
previously current thread.

5.5.3 Background Execution

GDB’s execution commands have two variants: the normal
foreground (synchronous) behavior, and a background
(asynchronous) behavior. In foreground execution, GDB waits for
the program to report that some thread has stopped before prompting for
another command. In background execution, GDB immediately gives
a command prompt so that you can issue other commands while your program runs.

If the target doesn’t support async mode, GDB issues an error
message if you attempt to use the background execution commands.

To specify background execution, add a & to the command. For example,
the background form of the continue command is continue&, or
just c&. The execution commands that accept background execution
are:

Background execution is especially useful in conjunction with non-stop
mode for debugging programs with multiple threads; see Non-Stop Mode.
However, you can also use these commands in the normal all-stop mode with
the restriction that you cannot issue another execution command until the
previous one finishes. Examples of commands that are valid in all-stop
mode while the program is running include help and info break.

You can interrupt your program while it is running in the background by
using the interrupt command.

interrupt

interrupt -a

Suspend execution of the running program. In all-stop mode,
interrupt stops the whole process, but in non-stop mode, it stops
only the current thread. To stop the whole program in non-stop mode,
use interrupt -a.

location specifies source lines; there are several ways of
writing them (see Specify Location), but the effect is always to
specify some source line.

Use the qualifier ‘thread thread-id’ with a breakpoint command
to specify that you only want GDB to stop the program when a
particular thread reaches this breakpoint. The thread-id specifier
is one of the thread identifiers assigned by GDB, shown
in the first column of the ‘info threads’ display.

If you do not specify ‘thread thread-id’ when you set a
breakpoint, the breakpoint applies to all threads of your
program.

You can use the thread qualifier on conditional breakpoints as
well; in this case, place ‘thread thread-id’ before or
after the breakpoint condition, like this:

(gdb) break frik.c:13 thread 28 if bartab > lim

Thread-specific breakpoints are automatically deleted when
GDB detects the corresponding thread is no longer in the
thread list. For example:

There are several ways for a thread to disappear, such as a regular
thread exit, but also when you detach from the process with the
detach command (see Debugging an Already-running
Process), or if GDB loses the remote connection
(see Remote Debugging), etc. Note that with some targets,
GDB is only able to detect a thread has exited when the user
explictly asks for the thread list with the info threads
command.

5.5.5 Interrupted System Calls

There is an unfortunate side effect when using GDB to debug
multi-threaded programs. If one thread stops for a
breakpoint, or for some other reason, and another thread is blocked in a
system call, then the system call may return prematurely. This is a
consequence of the interaction between multiple threads and the signals
that GDB uses to implement breakpoints and other events that
stop execution.

To handle this problem, your program should check the return value of
each system call and react appropriately. This is good programming
style anyways.

For example, do not write code like this:

sleep (10);

The call to sleep will return early if a different thread stops
at a breakpoint or for some other reason.

Instead, write this:

int unslept = 10;
while (unslept > 0)
unslept = sleep (unslept);

A system call is allowed to return early, so the system is still
conforming to its specification. But GDB does cause your
multi-threaded program to behave differently than it would without
GDB.

Also, GDB uses internal breakpoints in the thread library to
monitor certain events such as thread creation and thread destruction.
When such an event happens, a system call in another thread may return
prematurely, even though your program does not appear to stop.

5.5.6 Observer Mode

If you want to build on non-stop mode and observe program behavior
without any chance of disruption by GDB, you can set
variables to disable all of the debugger’s attempts to modify state,
whether by writing memory, inserting breakpoints, etc. These operate
at a low level, intercepting operations from all commands.

When all of these are set to off, then GDB is said to
be observer mode. As a convenience, the variable
observer can be set to disable these, plus enable non-stop
mode.

Note that GDB will not prevent you from making nonsensical
combinations of these settings. For instance, if you have enabled
may-insert-breakpoints but disabled may-write-memory,
then breakpoints that work by writing trap instructions into the code
stream will still not be able to be placed.

set observer on

set observer off

When set to on, this disables all the permission variables
below (except for insert-fast-tracepoints), plus enables
non-stop debugging. Setting this to off switches back to
normal debugging, though remaining in non-stop mode.

show observer

Show whether observer mode is on or off.

set may-write-registers on

set may-write-registers off

This controls whether GDB will attempt to alter the values of
registers, such as with assignment expressions in print, or the
jump command. It defaults to on.

show may-write-registers

Show the current permission to write registers.

set may-write-memory on

set may-write-memory off

This controls whether GDB will attempt to alter the contents
of memory, such as with assignment expressions in print. It
defaults to on.

show may-write-memory

Show the current permission to write memory.

set may-insert-breakpoints on

set may-insert-breakpoints off

This controls whether GDB will attempt to insert breakpoints.
This affects all breakpoints, including internal breakpoints defined
by GDB. It defaults to on.

show may-insert-breakpoints

Show the current permission to insert breakpoints.

set may-insert-tracepoints on

set may-insert-tracepoints off

This controls whether GDB will attempt to insert (regular)
tracepoints at the beginning of a tracing experiment. It affects only
non-fast tracepoints, fast tracepoints being under the control of
may-insert-fast-tracepoints. It defaults to on.

show may-insert-tracepoints

Show the current permission to insert tracepoints.

set may-insert-fast-tracepoints on

set may-insert-fast-tracepoints off

This controls whether GDB will attempt to insert fast
tracepoints at the beginning of a tracing experiment. It affects only
fast tracepoints, regular (non-fast) tracepoints being under the
control of may-insert-tracepoints. It defaults to on.

show may-insert-fast-tracepoints

Show the current permission to insert fast tracepoints.

set may-interrupt on

set may-interrupt off

This controls whether GDB will attempt to interrupt or stop
program execution. When this variable is off, the
interrupt command will have no effect, nor will
Ctrl-c. It defaults to on.

6 Running programs backward

When you are debugging a program, it is not unusual to realize that
you have gone too far, and some event of interest has already happened.
If the target environment supports it, GDB can allow you to
“rewind” the program by running it backward.

A target environment that supports reverse execution should be able
to “undo” the changes in machine state that have taken place as the
program was executing normally. Variables, registers etc. should
revert to their previous values. Obviously this requires a great
deal of sophistication on the part of the target environment; not
all target environments can support reverse execution.

When a program is executed in reverse, the instructions that
have most recently been executed are “un-executed”, in reverse
order. The program counter runs backward, following the previous
thread of execution in reverse. As each instruction is “un-executed”,
the values of memory and/or registers that were changed by that
instruction are reverted to their previous states. After executing
a piece of source code in reverse, all side effects of that code
should be “undone”, and all variables should be returned to their
prior values6.

If you are debugging in a target environment that supports
reverse execution, GDB provides the following commands.

reverse-continue [ignore-count]

rc [ignore-count]

Beginning at the point where your program last stopped, start executing
in reverse. Reverse execution will stop for breakpoints and synchronous
exceptions (signals), just like normal execution. Behavior of
asynchronous signals depends on the target environment.

reverse-step [count]

Run the program backward until control reaches the start of a
different source line; then stop it, and return control to GDB.

Like the step command, reverse-step will only stop
at the beginning of a source line. It “un-executes” the previously
executed source line. If the previous source line included calls to
debuggable functions, reverse-step will step (backward) into
the called function, stopping at the beginning of the last
statement in the called function (typically a return statement).

Also, as with the step command, if non-debuggable functions are
called, reverse-step will run thru them backward without stopping.

reverse-stepi [count]

Reverse-execute one machine instruction. Note that the instruction
to be reverse-executed is not the one pointed to by the program
counter, but the instruction executed prior to that one. For instance,
if the last instruction was a jump, reverse-stepi will take you
back from the destination of the jump to the jump instruction itself.

reverse-next [count]

Run backward to the beginning of the previous line executed in
the current (innermost) stack frame. If the line contains function
calls, they will be “un-executed” without stopping. Starting from
the first line of a function, reverse-next will take you back
to the caller of that function, before the function was called,
just as the normal next command would take you from the last
line of a function back to its return to its caller
7.

reverse-nexti [count]

Like nexti, reverse-nexti executes a single instruction
in reverse, except that called functions are “un-executed” atomically.
That is, if the previously executed instruction was a return from
another function, reverse-nexti will continue to execute
in reverse until the call to that function (from the current stack
frame) is reached.

reverse-finish

Just as the finish command takes you to the point where the
current function returns, reverse-finish takes you to the point
where it was called. Instead of ending up at the end of the current
function invocation, you end up at the beginning.

set exec-direction

Set the direction of target execution.

set exec-direction reverse

GDB will perform all execution commands in reverse, until the
exec-direction mode is changed to “forward”. Affected commands include
step, stepi, next, nexti, continue, and finish. The return
command cannot be used in reverse mode.

set exec-direction forward

GDB will perform all execution commands in the normal fashion.
This is the default.

7 Recording Inferior’s Execution and Replaying It

On some platforms, GDB provides a special process record
and replay target that can record a log of the process execution, and
replay it later with both forward and reverse execution commands.

When this target is in use, if the execution log includes the record
for the next instruction, GDB will debug in replay
mode. In the replay mode, the inferior does not really execute code
instructions. Instead, all the events that normally happen during
code execution are taken from the execution log. While code is not
really executed in replay mode, the values of registers (including the
program counter register) and the memory of the inferior are still
changed as they normally would. Their contents are taken from the
execution log.

If the record for the next instruction is not in the execution log,
GDB will debug in record mode. In this mode, the
inferior executes normally, and GDB records the execution log
for future replay.

The process record and replay target supports reverse execution
(see Reverse Execution), even if the platform on which the
inferior runs does not. However, the reverse execution is limited in
this case by the range of the instructions recorded in the execution
log. In other words, reverse execution on platforms that don’t
support it directly can only be done in the replay mode.

When debugging in the reverse direction, GDB will work in
replay mode as long as the execution log includes the record for the
previous instruction; otherwise, it will work in record mode, if the
platform supports reverse execution, or stop if not.

For architecture environments that support process record and replay,
GDB provides the following commands:

record method

This command starts the process record and replay target. The
recording method can be specified as parameter. Without a parameter
the command uses the full recording method. The following
recording methods are available:

full

Full record/replay recording using GDB’s software record and
replay implementation. This method allows replaying and reverse
execution.

btrace format

Hardware-supported instruction recording. This method does not record
data. Further, the data is collected in a ring buffer so old data will
be overwritten when the buffer is full. It allows limited reverse
execution. Variables and registers are not available during reverse
execution. In remote debugging, recording continues on disconnect.
Recorded data can be inspected after reconnecting. The recording may
be stopped using record stop.

The recording format can be specified as parameter. Without a parameter
the command chooses the recording format. The following recording
formats are available:

bts

Use the Branch Trace Store (BTS) recording format. In
this format, the processor stores a from/to record for each executed
branch in the btrace ring buffer.

pt

Use the Intel Processor Trace recording format. In this
format, the processor stores the execution trace in a compressed form
that is afterwards decoded by GDB.

The trace can be recorded with very low overhead. The compressed
trace format also allows small trace buffers to already contain a big
number of instructions compared to BTS.

Decoding the recorded execution trace, on the other hand, is more
expensive than decoding BTS trace. This is mostly due to the
increased number of instructions to process. You should increase the
buffer-size with care.

Not all recording formats may be available on all processors.

The process record and replay target can only debug a process that is
already running. Therefore, you need first to start the process with
the run or start commands, and then start the recording
with the record method command.

Displaced stepping (see displaced stepping)
will be automatically disabled when process record and replay target
is started. That’s because the process record and replay target
doesn’t support displaced stepping.

If the inferior is in the non-stop mode (see Non-Stop Mode) or in
the asynchronous execution mode (see Background Execution), not
all recording methods are available. The full recording method
does not support these two modes.

record stop

Stop the process record and replay target. When process record and
replay target stops, the entire execution log will be deleted and the
inferior will either be terminated, or will remain in its final state.

When you stop the process record and replay target in record mode (at
the end of the execution log), the inferior will be stopped at the
next instruction that would have been recorded. In other words, if
you record for a while and then stop recording, the inferior process
will be left in the same state as if the recording never happened.

On the other hand, if the process record and replay target is stopped
while in replay mode (that is, not at the end of the execution log,
but at some earlier point), the inferior process will become “live”
at that earlier state, and it will then be possible to continue the
usual “live” debugging of the process from that state.

When the inferior process exits, or GDB detaches from it,
process record and replay target will automatically stop itself.

record goto

Go to a specific location in the execution log. There are several
ways to specify the location to go to:

record goto begin

record goto start

Go to the beginning of the execution log.

record goto end

Go to the end of the execution log.

record goto n

Go to instruction number n in the execution log.

record save filename

Save the execution log to a file filename.
Default filename is gdb_record.process_id, where
process_id is the process ID of the inferior.

This command may not be available for all recording methods.

record restore filename

Restore the execution log from a file filename.
File must have been created with record save.

set record full insn-number-max limit

set record full insn-number-max unlimited

Set the limit of instructions to be recorded for the full
recording method. Default value is 200000.

If limit is a positive number, then GDB will start
deleting instructions from the log once the number of the record
instructions becomes greater than limit. For every new recorded
instruction, GDB will delete the earliest recorded
instruction to keep the number of recorded instructions at the limit.
(Since deleting recorded instructions loses information, GDB
lets you control what happens when the limit is reached, by means of
the stop-at-limit option, described below.)

If limit is unlimited or zero, GDB will never
delete recorded instructions from the execution log. The number of
recorded instructions is limited only by the available memory.

show record full insn-number-max

Show the limit of instructions to be recorded with the full
recording method.

set record full stop-at-limit

Control the behavior of the full recording method when the
number of recorded instructions reaches the limit. If ON (the
default), GDB will stop when the limit is reached for the
first time and ask you whether you want to stop the inferior or
continue running it and recording the execution log. If you decide
to continue recording, each new recorded instruction will cause the
oldest one to be deleted.

If this option is OFF, GDB will automatically delete the
oldest record to make room for each new one, without asking.

show record full stop-at-limit

Show the current setting of stop-at-limit.

set record full memory-query

Control the behavior when GDB is unable to record memory
changes caused by an instruction for the full recording method.
If ON, GDB will query whether to stop the inferior in that
case.

If this option is OFF (the default), GDB will automatically
ignore the effect of such instructions on memory. Later, when
GDB replays this execution log, it will mark the log of this
instruction as not accessible, and it will not affect the replay
results.

show record full memory-query

Show the current setting of memory-query.

The btrace record target does not trace data. As a
convenience, when replaying, GDB reads read-only memory off
the live program directly, assuming that the addresses of the
read-only areas don’t change. This for example makes it possible to
disassemble code while replaying, but not to print variables.
In some cases, being able to inspect variables might be useful.
You can use the following command for that:

set record btrace replay-memory-access

Control the behavior of the btrace recording method when
accessing memory during replay. If read-only (the default),
GDB will only allow accesses to read-only memory.
If read-write, GDB will allow accesses to read-only
and to read-write memory. Beware that the accessed memory corresponds
to the live target and not necessarily to the current replay
position.

set record btrace cpu identifier

Set the processor to be used for enabling workarounds for processor
errata when decoding the trace.

Processor errata are defects in processor operation, caused by its
design or manufacture. They can cause a trace not to match the
specification. This, in turn, may cause trace decode to fail.
GDB can detect erroneous trace packets and correct them, thus
avoiding the decoding failures. These corrections are known as
errata workarounds, and are enabled based on the processor on
which the trace was recorded.

By default, GDB attempts to detect the processor
automatically, and apply the necessary workarounds for it. However,
you may need to specify the processor if GDB does not yet
support it. This command allows you to do that, and also allows to
disable the workarounds.

The argument identifier identifies the CPU and is of the
form: vendor:procesor identifier. In addition,
there are two special identifiers, none and auto
(default).

The following vendor identifiers and corresponding processor
identifiers are currently supported:

intel

family/model[/stepping]

On GNU/Linux systems, the processor family, model, and
stepping can be obtained from /proc/cpuinfo.

If identifier is auto, enable errata workarounds for the
processor on which the trace was recorded. If identifier is
none, errata workarounds are disabled.

For example, when using an old GDB on a new system, decode
may fail because GDB does not support the new processor. It
often suffices to specify an older processor that GDB
supports.

Show the processor to be used for enabling trace decode errata
workarounds.

set record btrace bts buffer-size size

set record btrace bts buffer-size unlimited

Set the requested ring buffer size for branch tracing in BTS
format. Default is 64KB.

If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the BTS format.
The actually obtained buffer size may differ from the requested
size. Use the info record command to see the actual
buffer size for each thread that uses the btrace recording method and
the BTS format.

If limit is unlimited or zero, GDB will try to
allocate a buffer of 4MB.

Bigger buffers mean longer traces. On the other hand, GDB will
also need longer to process the branch trace data before it can be used.

show record btrace bts buffer-size size

Show the current setting of the requested ring buffer size for branch
tracing in BTS format.

If size is a positive number, then GDB will try to
allocate a buffer of at least size bytes for each new thread
that uses the btrace recording method and the Intel Processor Trace
format. The actually obtained buffer size may differ from the
requested size. Use the info record command to see the
actual buffer size for each thread.

If limit is unlimited or zero, GDB will try to
allocate a buffer of 4MB.

Bigger buffers mean longer traces. On the other hand, GDB will
also need longer to process the branch trace data before it can be used.

show record btrace pt buffer-size size

Show the current setting of the requested ring buffer size for branch
tracing in Intel Processor Trace format.

info record

Show various statistics about the recording depending on the recording
method:

full

For the full recording method, it shows the state of process
record and its in-memory execution log buffer, including:

Whether in record mode or replay mode.

Lowest recorded instruction number (counting from when the current execution log started recording instructions).

Highest recorded instruction number.

Current instruction about to be replayed (if in replay mode).

Number of instructions contained in the execution log.

Maximum number of instructions that may be contained in the execution log.

btrace

For the btrace recording method, it shows:

Recording format.

Number of instructions that have been recorded.

Number of blocks of sequential control-flow formed by the recorded
instructions.

Whether in record mode or replay mode.

For the bts recording format, it also shows:

Size of the perf ring buffer.

For the pt recording format, it also shows:

Size of the perf ring buffer.

record delete

When record target runs in replay mode (“in the past”), delete the
subsequent execution log and begin to record a new execution log starting
from the current address. This means you will abandon the previously
recorded “future” and begin recording a new “future”.

record instruction-history

Disassembles instructions from the recorded execution log. By
default, ten instructions are disassembled. This can be changed using
the set record instruction-history-size command. Instructions
are printed in execution order.

It can also print mixed source+disassembly if you specify the the
/m or /s modifier, and print the raw instructions in hex
as well as in symbolic form by specifying the /r modifier.

The current position marker is printed for the instruction at the
current program counter value. This instruction can appear multiple
times in the trace and the current position marker will be printed
every time. To omit the current position marker, specify the
/p modifier.

To better align the printed instructions when the trace contains
instructions from more than one function, the function name may be
omitted by specifying the /f modifier.

Speculatively executed instructions are prefixed with ‘?’. This
feature is not available for all recording formats.

There are several ways to specify what part of the execution log to
disassemble:

record instruction-history insn

Disassembles ten instructions starting from instruction number
insn.

record instruction-history insn, +/-n

Disassembles n instructions around instruction number
insn. If n is preceded with +, disassembles
n instructions after instruction number insn. If
n is preceded with -, disassembles n
instructions before instruction number insn.

record instruction-history

Disassembles ten more instructions after the last disassembly.

record instruction-history -

Disassembles ten more instructions before the last disassembly.

record instruction-history begin, end

Disassembles instructions beginning with instruction number
begin until instruction number end. The instruction
number end is included.

This command may not be available for all recording methods.

set record instruction-history-size size

set record instruction-history-size unlimited

Define how many instructions to disassemble in the record
instruction-history command. The default value is 10.
A size of unlimited means unlimited instructions.

show record instruction-history-size

Show how many instructions to disassemble in the record
instruction-history command.

record function-call-history

Prints the execution history at function granularity. It prints one
line for each sequence of instructions that belong to the same
function giving the name of that function, the source lines
for this instruction sequence (if the /l modifier is
specified), and the instructions numbers that form the sequence (if
the /i modifier is specified). The function names are indented
to reflect the call stack depth if the /c modifier is
specified. The /l, /i, and /c modifiers can be
given together.

By default, ten lines are printed. This can be changed using the
set record function-call-history-size command. Functions are
printed in execution order. There are several ways to specify what
to print:

record function-call-history func

Prints ten functions starting from function number func.

record function-call-history func, +/-n

Prints n functions around function number func. If
n is preceded with +, prints n functions after
function number func. If n is preceded with -,
prints n functions before function number func.

record function-call-history

Prints ten more functions after the last ten-line print.

record function-call-history -

Prints ten more functions before the last ten-line print.

record function-call-history begin, end

Prints functions beginning with function number begin until
function number end. The function number end is included.

This command may not be available for all recording methods.

set record function-call-history-size size

set record function-call-history-size unlimited

Define how many lines to print in the
record function-call-history command. The default value is 10.
A size of unlimited means unlimited lines.

show record function-call-history-size

Show how many lines to print in the
record function-call-history command.

8 Examining the Stack

When your program has stopped, the first thing you need to know is where it
stopped and how it got there.

Each time your program performs a function call, information about the call
is generated.
That information includes the location of the call in your program,
the arguments of the call,
and the local variables of the function being called.
The information is saved in a block of data called a stack frame.
The stack frames are allocated in a region of memory called the call
stack.

When your program stops, the GDB commands for examining the
stack allow you to see all of this information.

One of the stack frames is selected by GDB and many
GDB commands refer implicitly to the selected frame. In
particular, whenever you ask GDB for the value of a variable in
your program, the value is found in the selected frame. There are
special GDB commands to select whichever frame you are
interested in. See Selecting a Frame.

When your program stops, GDB automatically selects the
currently executing frame and describes it briefly, similar to the
frame command (see Information about a Frame).

8.1 Stack Frames

The call stack is divided up into contiguous pieces called stack
frames, or frames for short; each frame is the data associated
with one call to one function. The frame contains the arguments given
to the function, the function’s local variables, and the address at
which the function is executing.

When your program is started, the stack has only one frame, that of the
function main. This is called the initial frame or the
outermost frame. Each time a function is called, a new frame is
made. Each time a function returns, the frame for that function invocation
is eliminated. If a function is recursive, there can be many frames for
the same function. The frame for the function in which execution is
actually occurring is called the innermost frame. This is the most
recently created of all the stack frames that still exist.

Inside your program, stack frames are identified by their addresses. A
stack frame consists of many bytes, each of which has its own address; each
kind of computer has a convention for choosing one byte whose
address serves as the address of the frame. Usually this address is kept
in a register called the frame pointer register
(see $fp) while execution is going on in that frame.

GDB assigns numbers to all existing stack frames, starting with
zero for the innermost frame, one for the frame that called it,
and so on upward. These numbers do not really exist in your program;
they are assigned by GDB to give you a way of designating stack
frames in GDB commands.

Some compilers provide a way to compile functions so that they operate
without stack frames. (For example, the GCC option

‘-fomit-frame-pointer’

generates functions without a frame.)
This is occasionally done with heavily used library functions to save
the frame setup time. GDB has limited facilities for dealing
with these function invocations. If the innermost function invocation
has no stack frame, GDB nevertheless regards it as though
it had a separate frame, which is numbered zero as usual, allowing
correct tracing of the function call chain. However, GDB has
no provision for frameless functions elsewhere in the stack.

8.2 Backtraces

A backtrace is a summary of how your program got where it is. It shows one
line per frame, for many frames, starting with the currently executing
frame (frame zero), followed by its caller (frame one), and on up the
stack.

To print a backtrace of the entire stack, use the backtrace
command, or its alias bt. This command will print one line per
frame for frames in the stack. By default, all stack frames are
printed. You can stop the backtrace at any time by typing the system
interrupt character, normally Ctrl-c.

backtrace [args…]

bt [args…]

Print the backtrace of the entire stack. The optional args can
be one of the following:

n

n

Print only the innermost n frames, where n is a positive
number.

-n

-n

Print only the outermost n frames, where n is a positive
number.

full

Print the values of the local variables also. This can be combined
with a number to limit the number of frames shown.

no-filters

Do not run Python frame filters on this backtrace. See Frame Filter API, for more information. Additionally use disable frame-filter all to turn off all frame filters. This is only
relevant when GDB has been configured with Python
support.

hide

A Python frame filter might decide to “elide” some frames. Normally
such elided frames are still printed, but they are indented relative
to the filtered frames that cause them to be elided. The hide
option causes elided frames to not be printed at all.

The names where and info stack (abbreviated info s)
are additional aliases for backtrace.

In a multi-threaded program, GDB by default shows the
backtrace only for the current thread. To display the backtrace for
several or all of the threads, use the command thread apply
(see thread apply). For example, if you type thread
apply all backtrace, GDB will display the backtrace for all
the threads; this is handy when you debug a core dump of a
multi-threaded program.

Each line in the backtrace shows the frame number and the function name.
The program counter value is also shown—unless you use set
print address off. The backtrace also shows the source file name and
line number, as well as the arguments to the function. The program
counter value is omitted if it is at the beginning of the code for that
line number.

Here is an example of a backtrace. It was made with the command
‘bt 3’, so it shows the innermost three frames.

The display for frame zero does not begin with a program counter
value, indicating that your program has stopped at the beginning of the
code for line 993 of builtin.c.

The value of parameter data in frame 1 has been replaced by
…. By default, GDB prints the value of a parameter
only if it is a scalar (integer, pointer, enumeration, etc). See command
set print frame-arguments in Print Settings for more details
on how to configure the way function parameter values are printed.

If your program was compiled with optimizations, some compilers will
optimize away arguments passed to functions if those arguments are
never used after the call. Such optimizations generate code that
passes arguments through registers, but doesn’t store those arguments
in the stack frame. GDB has no way of displaying such
arguments in stack frames other than the innermost one. Here’s what
such a backtrace might look like:

The values of arguments that were not saved in their stack frames are
shown as ‘<optimized out>’.

If you need to display the values of such optimized-out arguments,
either deduce that from other variables whose values depend on the one
you are interested in, or recompile without optimizations.

Most programs have a standard user entry point—a place where system
libraries and startup code transition into user code. For C this is
main8.
When GDB finds the entry function in a backtrace
it will terminate the backtrace, to avoid tracing into highly
system-specific (and generally uninteresting) code.

If you need to examine the startup code, or limit the number of levels
in a backtrace, you can change this behavior:

set backtrace past-main

set backtrace past-main on

Backtraces will continue past the user entry point.

set backtrace past-main off

Backtraces will stop when they encounter the user entry point. This is the
default.

show backtrace past-main

Display the current user entry point backtrace policy.

set backtrace past-entry

set backtrace past-entry on

Backtraces will continue past the internal entry point of an application.
This entry point is encoded by the linker when the application is built,
and is likely before the user entry point main (or equivalent) is called.

set backtrace past-entry off

Backtraces will stop when they encounter the internal entry point of an
application. This is the default.

show backtrace past-entry

Display the current internal entry point backtrace policy.

set backtrace limit n

set backtrace limit 0

set backtrace limit unlimited

Limit the backtrace to n levels. A value of unlimited
or zero means unlimited levels.

show backtrace limit

Display the current limit on backtrace levels.

You can control how file names are displayed.

set filename-display

set filename-display relative

Display file names relative to the compilation directory. This is the default.

8.3 Selecting a Frame

Most commands for examining the stack and other data in your program work on
whichever stack frame is selected at the moment. Here are the commands for
selecting a stack frame; all of them finish by printing a brief description
of the stack frame just selected.

frame n

f n

Select frame number n. Recall that frame zero is the innermost
(currently executing) frame, frame one is the frame that called the
innermost one, and so on. The highest-numbered frame is the one for
main.

frame stack-addr [ pc-addr ]

f stack-addr [ pc-addr ]

Select the frame at address stack-addr. This is useful mainly if the
chaining of stack frames has been damaged by a bug, making it
impossible for GDB to assign numbers properly to all frames. In
addition, this can be useful when your program has multiple stacks and
switches between them. The optional pc-addr can also be given to
specify the value of PC for the stack frame.

up n

Move n frames up the stack; n defaults to 1. For positive
numbers n, this advances toward the outermost frame, to higher
frame numbers, to frames that have existed longer.

down n

Move n frames down the stack; n defaults to 1. For
positive numbers n, this advances toward the innermost frame, to
lower frame numbers, to frames that were created more recently.
You may abbreviate down as do.

All of these commands end by printing two lines of output describing the
frame. The first line shows the frame number, the function name, the
arguments, and the source file and line number of execution in that
frame. The second line shows the text of that source line.

After such a printout, the list command with no arguments
prints ten lines centered on the point of execution in the frame.
You can also edit the program at the point of execution with your favorite
editing program by typing edit.
See Printing Source Lines,
for details.

select-frame

The select-frame command is a variant of frame that does
not display the new frame after selecting it. This command is
intended primarily for use in GDB command scripts, where the
output might be unnecessary and distracting.

up-silently n

down-silently n

These two commands are variants of up and down,
respectively; they differ in that they do their work silently, without
causing display of the new frame. They are intended primarily for use
in GDB command scripts, where the output might be unnecessary and
distracting.

8.4 Information About a Frame

There are several other commands to print information about the selected
stack frame.

frame

f

When used without any argument, this command does not change which
frame is selected, but prints a brief description of the currently
selected stack frame. It can be abbreviated f. With an
argument, this command is used to select a stack frame.
See Selecting a Frame.

info frame

info f

This command prints a verbose description of the selected stack frame,
including:

the address of the frame

the address of the next frame down (called by this frame)

the address of the next frame up (caller of this frame)

the language in which the source code corresponding to this frame is written

the address of the frame’s arguments

the address of the frame’s local variables

the program counter saved in it (the address of execution in the caller frame)

which registers were saved in the frame

The verbose description is useful when
something has gone wrong that has made the stack format fail to fit
the usual conventions.

info frame addr

info f addr

Print a verbose description of the frame at address addr, without
selecting that frame. The selected frame remains unchanged by this
command. This requires the same kind of address (more than one for some
architectures) that you specify in the frame command.
See Selecting a Frame.

info args

Print the arguments of the selected frame, each on a separate line.

info locals

Print the local variables of the selected frame, each on a separate
line. These are all variables (declared either static or automatic)
accessible at the point of execution of the selected frame.

8.5 Management of Frame Filters.

Frame filters are Python based utilities to manage and decorate the
output of frames. See Frame Filter API, for further information.

Managing frame filters is performed by several commands available
within GDB, detailed here.

info frame-filter

Print a list of installed frame filters from all dictionaries, showing
their name, priority and enabled status.

disable frame-filter filter-dictionaryfilter-name

Disable a frame filter in the dictionary matching
filter-dictionary and filter-name. The
filter-dictionary may be all, global,
progspace, or the name of the object file where the frame filter
dictionary resides. When all is specified, all frame filters
across all dictionaries are disabled. The filter-name is the name
of the frame filter and is used when all is not the option for
filter-dictionary. A disabled frame-filter is not deleted, it
may be enabled again later.

enable frame-filter filter-dictionaryfilter-name

Enable a frame filter in the dictionary matching
filter-dictionary and filter-name. The
filter-dictionary may be all, global,
progspace or the name of the object file where the frame filter
dictionary resides. When all is specified, all frame filters across
all dictionaries are enabled. The filter-name is the name of the frame
filter and is used when all is not the option for
filter-dictionary.

Set the priority of a frame filter in the dictionary matching
filter-dictionary, and the frame filter name matching
filter-name. The filter-dictionary may be global,
progspace or the name of the object file where the frame filter
dictionary resides. The priority is an integer.

show frame-filter priority filter-dictionaryfilter-name

Show the priority of a frame filter in the dictionary matching
filter-dictionary, and the frame filter name matching
filter-name. The filter-dictionary may be global,
progspace or the name of the object file where the frame filter
dictionary resides.

9 Examining Source Files

GDB can print parts of your program’s source, since the debugging
information recorded in the program tells GDB what source files were
used to build it. When your program stops, GDB spontaneously prints
the line where it stopped. Likewise, when you select a stack frame
(see Selecting a Frame), GDB prints the line where
execution in that frame has stopped. You can print other portions of
source files by explicit command.

If you use GDB through its GNU Emacs interface, you may
prefer to use Emacs facilities to view source; see Using
GDB under GNU Emacs.

9.1 Printing Source Lines

To print lines from a source file, use the list command
(abbreviated l). By default, ten lines are printed.
There are several ways to specify what part of the file you want to
print; see Specify Location, for the full list.

Here are the forms of the list command most commonly used:

list linenum

Print lines centered around line number linenum in the
current source file.

list function

Print lines centered around the beginning of function
function.

list

Print more lines. If the last lines printed were printed with a
list command, this prints lines following the last lines
printed; however, if the last line printed was a solitary line printed
as part of displaying a stack frame (see Examining the
Stack), this prints lines centered around that line.

list -

Print lines just before the lines last printed.

By default, GDB prints ten source lines with any of these forms of
the list command. You can change this using set listsize:

Repeating a list command with RET discards the argument,
so it is equivalent to typing just list. This is more useful
than listing the same lines again. An exception is made for an
argument of ‘-’; that argument is preserved in repetition so that
each repetition moves up in the source file.

In general, the list command expects you to supply zero, one or two
locations. Locations specify source lines; there are several ways
of writing them (see Specify Location), but the effect is always
to specify some source line.

Here is a complete description of the possible arguments for list:

list location

Print lines centered around the line specified by location.

list first,last

Print lines from first to last. Both arguments are
locations. When a list command has two locations, and the
source file of the second location is omitted, this refers to
the same source file as the first location.

9.2 Specifying a Location

Several GDB commands accept arguments that specify a location
of your program’s code. Since GDB is a source-level
debugger, a location usually specifies some line in the source code.
Locations may be specified using three different formats:
linespec locations, explicit locations, or address locations.

9.2.1 Linespec Locations

A linespec is a colon-separated list of source location parameters such
as file name, function name, etc. Here are all the different ways of
specifying a linespec:

linenum

Specifies the line number linenum of the current source file.

-offset

+offset

Specifies the line offset lines before or after the current
line. For the list command, the current line is the last one
printed; for the breakpoint commands, this is the line at which
execution stopped in the currently selected stack frame
(see Frames, for a description of stack frames.) When
used as the second of the two linespecs in a list command,
this specifies the line offset lines up or down from the first
linespec.

filename:linenum

Specifies the line linenum in the source file filename.
If filename is a relative file name, then it will match any
source file name with the same trailing components. For example, if
filename is ‘gcc/expr.c’, then it will match source file
name of /build/trunk/gcc/expr.c, but not
/build/trunk/libcpp/expr.c or /build/trunk/gcc/x-expr.c.

function

Specifies the line that begins the body of the function function.
For example, in C, this is the line with the open brace.

By default, in C++ and Ada, function is interpreted as
specifying all functions named function in all scopes. For
C++, this means in all namespaces and classes. For Ada, this
means in all packages.

For example, assuming a program with C++ symbols named
A::B::func and B::func, both commands break func and break B::func set a breakpoint on both symbols.

Commands that accept a linespec let you override this with the
-qualified option. For example, break -qualified func sets a breakpoint on a free-function named func ignoring
any C++ class methods and namespace functions called func.

Specifies the line that begins the body of the function function
in the file filename. You only need the file name with a
function name to avoid ambiguity when there are identically named
functions in different source files.

label

Specifies the line at which the label named label appears
in the function corresponding to the currently selected stack frame.
If there is no current selected stack frame (for instance, if the inferior
is not running), then GDB will not search for a label.

-pstap|-probe-stap [objfile:[provider:]]name

The GNU/Linux tool SystemTap provides a way for
applications to embed static probes. See Static Probe Points, for more
information on finding and using static probes. This form of linespec
specifies the location of such a static probe.

If objfile is given, only probes coming from that shared library
or executable matching objfile as a regular expression are considered.
If provider is given, then only probes from that provider are considered.
If several probes match the spec, GDB will insert a breakpoint at
each one of those probes.

9.2.2 Explicit Locations

Explicit locations are useful when several functions, labels, or
file names have the same name (base name for files) in the program’s
sources. In these cases, explicit locations point to the source
line you meant more accurately and unambiguously. Also, using
explicit locations might be faster in large programs.

For example, the linespec ‘foo:bar’ may refer to a function bar
defined in the file named foo or the label bar in a function
named foo. GDB must search either the file system or
the symbol table to know.

The list of valid explicit location options is summarized in the
following table:

-source filename

The value specifies the source file name. To differentiate between
files with the same base name, prepend as many directories as is necessary
to uniquely identify the desired file, e.g., foo/bar/baz.c. Otherwise
GDB will use the first file it finds with the given base
name. This option requires the use of either -function or -line.

-function function

The value specifies the name of a function. Operations
on function locations unmodified by other options (such as -label
or -line) refer to the line that begins the body of the function.
In C, for example, this is the line with the open brace.

By default, in C++ and Ada, function is interpreted as
specifying all functions named function in all scopes. For
C++, this means in all namespaces and classes. For Ada, this
means in all packages.

For example, assuming a program with C++ symbols named
A::B::func and B::func, both commands break -function func and break -function B::func set a
breakpoint on both symbols.

You can use the -qualified flag to override this (see below).

-qualified

This flag makes GDB interpret a function name specified with
-function as a complete fully-qualified name.

For example, assuming a C++ program with symbols named
A::B::func and B::func, the break -qualified-function B::func command sets a breakpoint on B::func, only.

(Note: the -qualified option can precede a linespec as well
(see Linespec Locations), so the particular example above could be
simplified as break -qualified B::func.)

-label label

The value specifies the name of a label. When the function
name is not specified, the label is searched in the function of the currently
selected stack frame.

-line number

The value specifies a line offset for the location. The offset may either
be absolute (-line 3) or relative (-line +3), depending on
the command. When specified without any other options, the line offset is
relative to the current line.

9.2.3 Address Locations

Address locations indicate a specific program address. They have
the generalized form *address.

For line-oriented commands, such as list and edit, this
specifies a source line that contains address. For break and
other breakpoint-oriented commands, this can be used to set breakpoints in
parts of your program which do not have debugging information or
source files.

Here address may be any expression valid in the current working
language (see working language) that specifies a code
address. In addition, as a convenience, GDB extends the
semantics of expressions used in locations to cover several situations
that frequently occur during debugging. Here are the various forms
of address:

expression

Any expression valid in the current working language.

funcaddr

An address of a function or procedure derived from its name. In C,
C++, Objective-C, Fortran, minimal, and assembly, this is
simply the function’s name function (and actually a special case
of a valid expression). In Pascal and Modula-2, this is
&function. In Ada, this is function'Address
(although the Pascal form also works).

This form specifies the address of the function’s first instruction,
before the stack frame and arguments have been set up.

'filename':funcaddr

Like funcaddr above, but also specifies the name of the source
file explicitly. This is useful if the name of the function does not
specify the function unambiguously, e.g., if there are several
functions with identical names in different source files.

9.3 Editing Source Files

To edit the lines in a source file, use the edit command.
The editing program of your choice
is invoked with the current line set to
the active line in the program.
Alternatively, there are several ways to specify what part of the file you
want to print if you want to see other parts of the program:

edit location

Edit the source file specified by location. Editing starts at
that location, e.g., at the specified source line of the
specified file. See Specify Location, for all the possible forms
of the location argument; here are the forms of the edit
command most commonly used:

edit number

Edit the current source file with number as the active line number.

edit function

Edit the file containing function at the beginning of its definition.

9.3.1 Choosing your Editor

You can customize GDB to use any editor you want
9.
By default, it is /bin/ex, but you can change this
by setting the environment variable EDITOR before using
GDB. For example, to configure GDB to use the
vi editor, you could use these commands with the sh shell:

9.4 Searching Source Files

There are two commands for searching through the current source file for a
regular expression.

forward-search regexp

search regexp

The command ‘forward-search regexp’ checks each line,
starting with the one following the last line listed, for a match for
regexp. It lists the line that is found. You can use the
synonym ‘search regexp’ or abbreviate the command name as
fo.

reverse-search regexp

The command ‘reverse-search regexp’ checks each line, starting
with the one before the last line listed and going backward, for a match
for regexp. It lists the line that is found. You can abbreviate
this command as rev.

9.5 Specifying Source Directories

Executable programs sometimes do not record the directories of the source
files from which they were compiled, just the names. Even when they do,
the directories could be moved between the compilation and your debugging
session. GDB has a list of directories to search for source files;
this is called the source path. Each time GDB wants a source file,
it tries all the directories in the list, in the order they are present
in the list, until it finds a file with the desired name.

For example, suppose an executable references the file
/usr/src/foo-1.0/lib/foo.c, and our source path is
/mnt/cross. The file is first looked up literally; if this
fails, /mnt/cross/usr/src/foo-1.0/lib/foo.c is tried; if this
fails, /mnt/cross/foo.c is opened; if this fails, an error
message is printed. GDB does not look up the parts of the
source file name, such as /mnt/cross/src/foo-1.0/lib/foo.c.
Likewise, the subdirectories of the source path are not searched: if
the source path is /mnt/cross, and the binary refers to
foo.c, GDB would not find it under
/mnt/cross/usr/src/foo-1.0/lib.

Plain file names, relative file names with leading directories, file
names containing dots, etc. are all treated as described above; for
instance, if the source path is /mnt/cross, and the source file
is recorded as ../lib/foo.c, GDB would first try
../lib/foo.c, then /mnt/cross/../lib/foo.c, and after
that—/mnt/cross/foo.c.

Note that the executable search path is not used to locate the
source files.

Whenever you reset or rearrange the source path, GDB clears out
any information it has cached about where source files are found and where
each line is in the file.

When you start GDB, its source path includes only ‘cdir’
and ‘cwd’, in that order.
To add other directories, use the directory command.

The search path is used to find both program source files and GDB
script files (read using the ‘-command’ option and ‘source’ command).

In addition to the source path, GDB provides a set of commands
that manage a list of source path substitution rules. A substitution
rule specifies how to rewrite source directories stored in the program’s
debug information in case the sources were moved to a different
directory between compilation and debugging. A rule is made of
two strings, the first specifying what needs to be rewritten in
the path, and the second specifying how it should be rewritten.
In set substitute-path, we name these two parts from and
to respectively. GDB does a simple string replacement
of from with to at the start of the directory part of the
source file name, and uses that result instead of the original file
name to look up the sources.

Using the previous example, suppose the foo-1.0 tree has been
moved from /usr/src to /mnt/cross, then you can tell
GDB to replace /usr/src in all source path names with
/mnt/cross. The first lookup will then be
/mnt/cross/foo-1.0/lib/foo.c in place of the original location
of /usr/src/foo-1.0/lib/foo.c. To define a source path
substitution rule, use the set substitute-path command
(see set substitute-path).

To avoid unexpected substitution results, a rule is applied only if the
from part of the directory name ends at a directory separator.
For instance, a rule substituting /usr/source into
/mnt/cross will be applied to /usr/source/foo-1.0 but
not to /usr/sourceware/foo-2.0. And because the substitution
is applied only at the beginning of the directory name, this rule will
not be applied to /root/usr/source/baz.c either.

In many cases, you can achieve the same result using the directory
command. However, set substitute-path can be more efficient in
the case where the sources are organized in a complex tree with multiple
subdirectories. With the directory command, you need to add each
subdirectory of your project. If you moved the entire tree while
preserving its internal organization, then set substitute-path
allows you to direct the debugger to all the sources with one single
command.

set substitute-path is also more than just a shortcut command.
The source path is only used if the file at the original location no
longer exists. On the other hand, set substitute-path modifies
the debugger behavior to look at the rewritten location instead. So, if
for any reason a source file that is not relevant to your executable is
located at the original location, a substitution rule is the only
method available to point GDB at the new location.

You can configure a default source path substitution rule by
configuring GDB with the
‘--with-relocated-sources=dir’ option. The dir
should be the name of a directory under GDB’s configured
prefix (set with ‘--prefix’ or ‘--exec-prefix’), and
directory names in debug information under dir will be adjusted
automatically if the installed GDB is moved to a new
location. This is useful if GDB, libraries or executables
with debug information and corresponding source code are being moved
together.

directory dirname …

dir dirname …

Add directory dirname to the front of the source path. Several
directory names may be given to this command, separated by ‘:’
(‘;’ on MS-DOS and MS-Windows, where ‘:’ usually appears as
part of absolute file names) or
whitespace. You may specify a directory that is already in the source
path; this moves it forward, so GDB searches it sooner.

You can use the string ‘$cdir’ to refer to the compilation
directory (if one is recorded), and ‘$cwd’ to refer to the current
working directory. ‘$cwd’ is not the same as ‘.’—the former
tracks the current working directory as it changes during your GDB
session, while the latter is immediately expanded to the current
directory at the time you add an entry to the source path.

directory

Reset the source path to its default value (‘$cdir:$cwd’ on Unix systems). This requires confirmation.

set directories path-list

Set the source path to path-list.
‘$cdir:$cwd’ are added if missing.

show directories

Print the source path: show which directories it contains.

set substitute-path fromto

Define a source path substitution rule, and add it at the end of the
current list of existing substitution rules. If a rule with the same
from was already defined, then the old rule is also deleted.

For example, if the file /foo/bar/baz.c was moved to
/mnt/cross/baz.c, then the command

(gdb) set substitute-path /foo/bar /mnt/cross

will tell GDB to replace ‘/foo/bar’ with
‘/mnt/cross’, which will allow GDB to find the file
baz.c even though it was moved.

In the case when more than one substitution rule have been defined,
the rules are evaluated one by one in the order where they have been
defined. The first one matching, if any, is selected to perform
the substitution.

GDB would then rewrite /usr/src/include/defs.h into
/mnt/include/defs.h by using the first rule. However, it would
use the second rule to rewrite /usr/src/lib/foo.c into
/mnt/src/lib/foo.c.

unset substitute-path [path]

If a path is specified, search the current list of substitution rules
for a rule that would rewrite that path. Delete that rule if found.
A warning is emitted by the debugger if no rule could be found.

If no path is specified, then all substitution rules are deleted.

show substitute-path [path]

If a path is specified, then print the source path substitution rule
which would rewrite that path, if any.

If no path is specified, then print all existing source path substitution
rules.

If your source path is cluttered with directories that are no longer of
interest, GDB may sometimes cause confusion by finding the wrong
versions of source. You can correct the situation as follows:

Use directory with no argument to reset the source path to its default value.

Use directory with suitable arguments to reinstall the
directories you want in the source path. You can add all the
directories in one command.

9.6 Source and Machine Code

You can use the command info line to map source lines to program
addresses (and vice versa), and the command disassemble to display
a range of addresses as machine instructions. You can use the command
set disassemble-next-line to set whether to disassemble next
source line when execution stops. When run under GNU Emacs
mode, the info line command causes the arrow to point to the
line specified. Also, info line prints addresses in symbolic form as
well as hex.

info line

info line location

Print the starting and ending addresses of the compiled code for
source line location. You can specify source lines in any of
the ways documented in Specify Location. With no location
information about the current source line is printed.

For example, we can use info line to discover the location of
the object code for the first line of function
m4_changequote:

(gdb) info line m4_changequote
Line 895 of "builtin.c" starts at pc 0x634c <m4_changequote> and \
ends at 0x6350 <m4_changequote+4>.

We can also inquire (using *addr as the form for
location) what source line covers a particular address:

(gdb) info line *0x63ff
Line 926 of "builtin.c" starts at pc 0x63e4 <m4_changequote+152> and \
ends at 0x6404 <m4_changequote+184>.

After info line, the default address for the x command
is changed to the starting address of the line, so that ‘x/i’ is
sufficient to begin examining the machine code (see Examining Memory). Also, this address is saved as the value of the
convenience variable $_ (see Convenience
Variables).

After info line, using info line again without
specifying a location will display information about the next source
line.

disassemble

disassemble /m

disassemble /s

disassemble /r

This specialized command dumps a range of memory as machine
instructions. It can also print mixed source+disassembly by specifying
the /m or /s modifier and print the raw instructions in hex
as well as in symbolic form by specifying the /r modifier.
The default memory range is the function surrounding the
program counter of the selected frame. A single argument to this
command is a program counter value; GDB dumps the function
surrounding this value. When two arguments are given, they should
be separated by a comma, possibly surrounded by whitespace. The
arguments specify a range of addresses to dump, in one of two forms:

start,end

the addresses from start (inclusive) to end (exclusive)

start,+length

the addresses from start (inclusive) to
start+length (exclusive).

When 2 arguments are specified, the name of the function is also
printed (since there could be several functions in the given range).

The argument(s) can be any expression yielding a numeric value, such as
‘0x32c4’, ‘&main+10’ or ‘$pc - 8’.

If the range of memory being disassembled contains current program counter,
the instruction at that location is shown with a => marker.

The following example shows the disassembly of a range of addresses of
HP PA-RISC 2.0 code:

The /m option is deprecated as its output is not useful when
there is either inlined code or re-ordered code.
The /s option is the preferred choice.
Here is an example for AMD x86-64 showing the difference between
/m output and /s output.
This example has one inline function defined in a header file,
and the code is compiled with ‘-O2’ optimization.
Note how the /m output is missing the disassembly of
several instructions that are present in the /s output.

Addresses cannot be specified as a location (see Specify Location).
So, for example, if you want to disassemble function bar
in file foo.c, you must type ‘disassemble 'foo.c'::bar’
and not ‘disassemble foo.c:bar’.

Some architectures have more than one commonly-used set of instruction
mnemonics or other syntax.

For programs that were dynamically linked and use shared libraries,
instructions that call functions or branch to locations in the shared
libraries might show a seemingly bogus location—it’s actually a
location of the relocation table. On some architectures, GDB
might be able to resolve these to actual function names.

set disassembler-options option1[,option2…]

This command controls the passing of target specific information to
the disassembler. For a list of valid options, please refer to the
-M/--disassembler-options section of the ‘objdump’
manual and/or the output of objdump --help
(see objdump in The GNU Binary Utilities).
The default value is the empty string.

If it is necessary to specify more than one disassembler option, then
multiple options can be placed together into a comma separated list.
Currently this command is only supported on targets ARM, MIPS, PowerPC
and S/390.

show disassembler-options

Show the current setting of the disassembler options.

set disassembly-flavor instruction-set

Select the instruction set to use when disassembling the
program via the disassemble or x/i commands.

Currently this command is only defined for the Intel x86 family. You
can set instruction-set to either intel or att.
The default is att, the AT&T flavor used by default by Unix
assemblers for x86-based targets.

show disassembly-flavor

Show the current setting of the disassembly flavor.

set disassemble-next-line

show disassemble-next-line

Control whether or not GDB will disassemble the next source
line or instruction when execution stops. If ON, GDB will
display disassembly of the next source line when execution of the
program being debugged stops. This is in addition to
displaying the source line itself, which GDB always does if
possible. If the next source line cannot be displayed for some reason
(e.g., if GDB cannot find the source file, or there’s no line
info in the debug info), GDB will display disassembly of the
next instruction instead of showing the next source line. If
AUTO, GDB will display disassembly of next instruction only
if the source line cannot be displayed. This setting causes
GDB to display some feedback when you step through a function
with no line info or whose source file is unavailable. The default is
OFF, which means never display the disassembly of the next line or
instruction.

10 Examining Data

The usual way to examine data in your program is with the print
command (abbreviated p), or its synonym inspect. It
evaluates and prints the value of an expression of the language your
program is written in (see Using GDB with
Different Languages). It may also print the expression using a
Python-based pretty-printer (see Pretty Printing).

print expr

print /fexpr

expr is an expression (in the source language). By default the
value of expr is printed in a format appropriate to its data type;
you can choose a different format by specifying ‘/f’, where
f is a letter specifying the format; see Output
Formats.

print

print /f

If you omit expr, GDB displays the last value again (from the
value history; see Value History). This allows you to
conveniently inspect the same value in an alternative format.

A more low-level way of examining data is with the x command.
It examines data in memory at a specified address and prints it in a
specified format. See Examining Memory.

If you are interested in information about types, or about how the
fields of a struct or a class are declared, use the ptype exp
command rather than print. See Examining the Symbol
Table.

Another way of examining values of expressions and type information is
through the Python extension command explore (available only if
the GDB build is configured with --with-python). It
offers an interactive way to start at the highest level (or, the most
abstract level) of the data type of an expression (or, the data type
itself) and explore all the way down to leaf scalar values/fields
embedded in the higher level data types.

explore arg

arg is either an expression (in the source language), or a type
visible in the current context of the program being debugged.

The working of the explore command can be illustrated with an
example. If a data type struct ComplexStruct is defined in your
C program as

then, the value of the variable cs can be explored using the
explore command as follows.

(gdb) explore cs
The value of `cs' is a struct/class of type `struct ComplexStruct' with
the following fields:
ss_p = <Enter 0 to explore this field of type `struct SimpleStruct *'>
arr = <Enter 1 to explore this field of type `int [10]'>
Enter the field number of choice:

Since the fields of cs are not scalar values, you are being
prompted to chose the field you want to explore. Let’s say you choose
the field ss_p by entering 0. Then, since this field is a
pointer, you will be asked if it is pointing to a single value. From
the declaration of cs above, it is indeed pointing to a single
value, hence you enter y. If you enter n, then you will
be asked if it were pointing to an array of values, in which case this
field will be explored as if it were an array.

`cs.ss_p' is a pointer to a value of type `struct SimpleStruct'
Continue exploring it as a pointer to a single value [y/n]: y
The value of `*(cs.ss_p)' is a struct/class of type `struct
SimpleStruct' with the following fields:
i = 10 .. (Value of type `int')
d = 1.1100000000000001 .. (Value of type `double')
Press enter to return to parent value:

If the field arr of cs was chosen for exploration by
entering 1 earlier, then since it is as array, you will be
prompted to enter the index of the element in the array that you want
to explore.

`cs.arr' is an array of `int'.
Enter the index of the element you want to explore in `cs.arr': 5
`(cs.arr)[5]' is a scalar value of type `int'.
(cs.arr)[5] = 4
Press enter to return to parent value:

In general, at any stage of exploration, you can go deeper towards the
leaf values by responding to the prompts appropriately, or hit the
return key to return to the enclosing data structure (the higher
level data structure).

Similar to exploring values, you can use the explore command to
explore types. Instead of specifying a value (which is typically a
variable name or an expression valid in the current context of the
program being debugged), you specify a type name. If you consider the
same example as above, your can explore the type
struct ComplexStruct by passing the argument
struct ComplexStruct to the explore command.

(gdb) explore struct ComplexStruct

By responding to the prompts appropriately in the subsequent interactive
session, you can explore the type struct ComplexStruct in a
manner similar to how the value cs was explored in the above
example.

The explore command also has two sub-commands,
explore value and explore type. The former sub-command is
a way to explicitly specify that value exploration of the argument is
being invoked, while the latter is a way to explicitly specify that type
exploration of the argument is being invoked.

explore value expr

This sub-command of explore explores the value of the
expression expr (if expr is an expression valid in the
current context of the program being debugged). The behavior of this
command is identical to that of the behavior of the explore
command being passed the argument expr.

explore type arg

This sub-command of explore explores the type of arg (if
arg is a type visible in the current context of program being
debugged), or the type of the value/expression arg (if arg
is an expression valid in the current context of the program being
debugged). If arg is a type, then the behavior of this command is
identical to that of the explore command being passed the
argument arg. If arg is an expression, then the behavior of
this command will be identical to that of the explore command
being passed the type of arg as the argument.

10.1 Expressions

print and many other GDB commands accept an expression and
compute its value. Any kind of constant, variable or operator defined
by the programming language you are using is valid in an expression in
GDB. This includes conditional expressions, function calls,
casts, and string constants. It also includes preprocessor macros, if
you compiled your program to include this information; see
Compilation.

GDB supports array constants in expressions input by
the user. The syntax is {element, element…}. For example,
you can use the command print {1, 2, 3} to create an array
of three integers. If you pass an array to a function or assign it
to a program variable, GDB copies the array to memory that
is malloced in the target program.

Because C is so widespread, most of the expressions shown in examples in
this manual are in C. See Using GDB with Different
Languages, for information on how to use expressions in other
languages.

In this section, we discuss operators that you can use in GDB
expressions regardless of your programming language.

Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer in order to examine a structure
at that address in memory.

GDB supports these operators, in addition to those common
to programming languages:

@

‘@’ is a binary operator for treating parts of memory as arrays.
See Artificial Arrays, for more information.

::

‘::’ allows you to specify a variable in terms of the file or
function where it is defined. See Program Variables.

{type} addr

Refers to an object of type type stored at address addr in
memory. The address addr may be any expression whose value is
an integer or pointer (but parentheses are required around binary
operators, just as in a cast). This construct is allowed regardless
of what kind of data is normally supposed to reside at addr.

10.2 Ambiguous Expressions

Expressions can sometimes contain some ambiguous elements. For instance,
some programming languages (notably Ada, C++ and Objective-C) permit
a single function name to be defined several times, for application in
different contexts. This is called overloading. Another example
involving Ada is generics. A generic package is similar to C++
templates and is typically instantiated several times, resulting in
the same function name being defined in different contexts.

In some cases and depending on the language, it is possible to adjust
the expression to remove the ambiguity. For instance in C++, you
can specify the signature of the function you want to break on, as in
break function(types). In Ada, using the fully
qualified name of your function often makes the expression unambiguous
as well.

When an ambiguity that needs to be resolved is detected, the debugger
has the capability to display a menu of numbered choices for each
possibility, and then waits for the selection with the prompt ‘>’.
The first option is always ‘[0] cancel’, and typing 0 RET
aborts the current command. If the command in which the expression was
used allows more than one choice to be selected, the next option in the
menu is ‘[1] all’, and typing 1 RET selects all possible
choices.

For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol String::after.
We choose three particular definitions of that function name:

This option allows you to adjust the debugger behavior when an expression
is ambiguous.

By default, mode is set to all. If the command with which
the expression is used allows more than one choice, then GDB
automatically selects all possible choices. For instance, inserting
a breakpoint on a function using an ambiguous name results in a breakpoint
inserted on each possible match. However, if a unique choice must be made,
then GDB uses the menu to help you disambiguate the expression.
For instance, printing the address of an overloaded function will result
in the use of the menu.

When mode is set to ask, the debugger always uses the menu
when an ambiguity is detected.

Finally, when mode is set to cancel, the debugger reports
an error due to the ambiguity and the command is aborted.

10.3 Program Variables

The most common kind of expression to use is the name of a variable
in your program.

Variables in expressions are understood in the selected stack frame
(see Selecting a Frame); they must be either:

global (or file-static)

or

visible according to the scope rules of the
programming language from the point of execution in that frame

This means that in the function

foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}

you can examine and use the variable a whenever your program is
executing within the function foo, but you can only use or
examine the variable b while your program is executing inside
the block where b is declared.

There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file by
using the colon-colon (::) notation:

file::variablefunction::variable

Here file or function is the name of the context for the
static variable. In the case of file names, you can use quotes to
make sure GDB parses the file name as a single word—for example,
to print a global value of x defined in f2.c:

(gdb) p 'f2.c'::x

The :: notation is normally used for referring to
static variables, since you typically disambiguate uses of local variables
in functions by selecting the appropriate frame and using the
simple name of the variable. However, you may also use this notation
to refer to local variables in frames enclosing the selected frame:

These uses of ‘::’ are very rarely in conflict with the very
similar use of the same notation in C++. When they are in
conflict, the C++ meaning takes precedence; however, this can be
overridden by quoting the file or function name with single quotes.

For example, suppose the program is stopped in a method of a class
that has a field named includefile, and there is also an
include file named includefile that defines a variable,
some_global.

Warning: Occasionally, a local variable may appear to have the
wrong value at certain points in a function—just after entry to a new
scope, and just before exit.

You may see this problem when you are stepping by machine instructions.
This is because, on most machines, it takes more than one instruction to
set up a stack frame (including local variable definitions); if you are
stepping by machine instructions, variables may appear to have the wrong
values until the stack frame is completely built. On exit, it usually
also takes more than one machine instruction to destroy a stack frame;
after you begin stepping through that group of instructions, local
variable definitions may be gone.

This may also happen when the compiler does significant optimizations.
To be sure of always seeing accurate values, turn off all optimization
when compiling.

Another possible effect of compiler optimizations is to optimize
unused variables out of existence, or assign variables to registers (as
opposed to memory addresses). Depending on the support for such cases
offered by the debug info format used by the compiler, GDB
might not be able to display values for such local variables. If that
happens, GDB will print a message like this:

No symbol "foo" in current context.

To solve such problems, either recompile without optimizations, or use a
different debug info format, if the compiler supports several such
formats. See Compilation, for more information on choosing compiler
options. See C and C++, for more information about debug
info formats that are best suited to C++ programs.

If you ask to print an object whose contents are unknown to
GDB, e.g., because its data type is not completely specified
by the debug information, GDB will say ‘<incomplete
type>’. See incomplete type, for more about this.

If you try to examine or use the value of a (global) variable for
which GDB has no type information, e.g., because the program
includes no debug information, GDB displays an error message.
See unknown type, for more about unknown types. If you
cast the variable to its declared type, GDB gets the
variable’s value using the cast-to type as the variable’s type. For
example, in a C program:

If you append @entry string to a function parameter name you get its
value at the time the function got called. If the value is not available an
error message is printed. Entry values are available only with some compilers.
Entry values are normally also printed at the function parameter list according
to set print entry-values.

Strings are identified as arrays of char values without specified
signedness. Arrays of either signed char or unsigned char get
printed as arrays of 1 byte sized integers. -fsigned-char or
-funsigned-charGCC options have no effect as GDB
defines literal string type "char" as char without a sign.
For program code

10.4 Artificial Arrays

It is often useful to print out several successive objects of the
same type in memory; a section of an array, or an array of
dynamically determined size for which only a pointer exists in the
program.

You can do this by referring to a contiguous span of memory as an
artificial array, using the binary operator ‘@’. The left
operand of ‘@’ should be the first element of the desired array
and be an individual object. The right operand should be the desired length
of the array. The result is an array value whose elements are all of
the type of the left argument. The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on. Here is an
example. If a program says

int *array = (int *) malloc (len * sizeof (int));

you can print the contents of array with

p *array@len

The left operand of ‘@’ must reside in memory. Array values made
with ‘@’ in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(see Value History), after printing one out.

Another way to create an artificial array is to use a cast.
This re-interprets a value as if it were an array.
The value need not be in memory:

(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}

As a convenience, if you leave the array length out (as in
‘(type[])value’) GDB calculates the size to fill
the value (as ‘sizeof(value)/sizeof(type)’:

(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}

Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent—for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is
to use a convenience variable (see Convenience
Variables) as a counter in an expression that prints the first
interesting value, and then repeat that expression via RET. For
instance, suppose you have an array dtab of pointers to
structures, and you are interested in the values of a field fv
in each structure. Here is an example of what you might type:

10.5 Output Formats

By default, GDB prints a value according to its data type. Sometimes
this is not what you want. For example, you might want to print a number
in hex, or a pointer in decimal. Or you might want to view data in memory
at a certain address as a character string or as an instruction. To do
these things, specify an output format when you print a value.

The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the
print command with a slash and a format letter. The format
letters supported are:

x

Regard the bits of the value as an integer, and print the integer in
hexadecimal.

Regard as an integer and print it as a character constant. This
prints both the numerical value and its character representation. The
character representation is replaced with the octal escape ‘\nnn’
for characters outside the 7-bit ASCII range.

Without this format, GDB displays char,
unsigned char, and signed char data as character
constants. Single-byte members of vectors are displayed as integer
data.

f

Regard the bits of the value as a floating point number and print
using typical floating point syntax.

s

Regard as a string, if possible. With this format, pointers to single-byte
data are displayed as null-terminated strings and arrays of single-byte data
are displayed as fixed-length strings. Other values are displayed in their
natural types.

Without this format, GDB displays pointers to and arrays of
char, unsigned char, and signed char as
strings. Single-byte members of a vector are displayed as an integer
array.

z

Like ‘x’ formatting, the value is treated as an integer and
printed as hexadecimal, but leading zeros are printed to pad the value
to the size of the integer type.

r

Print using the ‘raw’ formatting. By default, GDB will
use a Python-based pretty-printer, if one is available (see Pretty Printing). This typically results in a higher-level display of the
value’s contents. The ‘r’ format bypasses any Python
pretty-printer which might exist.

For example, to print the program counter in hex (see Registers), type

p/x $pc

Note that no space is required before the slash; this is because command
names in GDB cannot contain a slash.

To reprint the last value in the value history with a different format,
you can use the print command with just a format and no
expression. For example, ‘p/x’ reprints the last value in hex.

10.6 Examining Memory

You can use the command x (for “examine”) to examine memory in
any of several formats, independently of your program’s data types.

x/nfuaddr

x addr

x

Use the x command to examine memory.

n, f, and u are all optional parameters that specify how
much memory to display and how to format it; addr is an
expression giving the address where you want to start displaying memory.
If you use defaults for nfu, you need not type the slash ‘/’.
Several commands set convenient defaults for addr.

n, the repeat count

The repeat count is a decimal integer; the default is 1. It specifies
how much memory (counting by units u) to display. If a negative
number is specified, memory is examined backward from addr.

f, the display format

The display format is one of the formats used by print
(‘x’, ‘d’, ‘u’, ‘o’, ‘t’, ‘a’, ‘c’,
‘f’, ‘s’), and in addition ‘i’ (for machine instructions).
The default is ‘x’ (hexadecimal) initially. The default changes
each time you use either x or print.

u, the unit size

The unit size is any of

b

Bytes.

h

Halfwords (two bytes).

w

Words (four bytes). This is the initial default.

g

Giant words (eight bytes).

Each time you specify a unit size with x, that size becomes the
default unit the next time you use x. For the ‘i’ format,
the unit size is ignored and is normally not written. For the ‘s’ format,
the unit size defaults to ‘b’, unless it is explicitly given.
Use x /hs to display 16-bit char strings and x /ws to display
32-bit strings. The next use of x /s will again display 8-bit strings.
Note that the results depend on the programming language of the
current compilation unit. If the language is C, the ‘s’
modifier will use the UTF-16 encoding while ‘w’ will use
UTF-32. The encoding is set by the programming language and cannot
be altered.

addr, starting display address

addr is the address where you want GDB to begin displaying
memory. The expression need not have a pointer value (though it may);
it is always interpreted as an integer address of a byte of memory.
See Expressions, for more information on expressions. The default for
addr is usually just after the last address examined—but several
other commands also set the default address: info breakpoints (to
the address of the last breakpoint listed), info line (to the
starting address of a line), and print (if you use it to display
a value from memory).

For example, ‘x/3uh 0x54320’ is a request to display three halfwords
(h) of memory, formatted as unsigned decimal integers (‘u’),
starting at address 0x54320. ‘x/4xw $sp’ prints the four
words (‘w’) of memory above the stack pointer (here, ‘$sp’;
see Registers) in hexadecimal (‘x’).

You can also specify a negative repeat count to examine memory backward
from the given address. For example, ‘x/-3uh 0x54320’ prints three
halfwords (h) at 0x54314, 0x54328, and 0x5431c.

Since the letters indicating unit sizes are all distinct from the
letters specifying output formats, you do not have to remember whether
unit size or format comes first; either order works. The output
specifications ‘4xw’ and ‘4wx’ mean exactly the same thing.
(However, the count n must come first; ‘wx4’ does not work.)

Even though the unit size u is ignored for the formats ‘s’
and ‘i’, you might still want to use a count n; for example,
‘3i’ specifies that you want to see three machine instructions,
including any operands. For convenience, especially when used with
the display command, the ‘i’ format also prints branch delay
slot instructions, if any, beyond the count specified, which immediately
follow the last instruction that is within the count. The command
disassemble gives an alternative way of inspecting machine
instructions; see Source and Machine Code.

If a negative repeat count is specified for the formats ‘s’ or ‘i’,
the command displays null-terminated strings or instructions before the given
address as many as the absolute value of the given number. For the ‘i’
format, we use line number information in the debug info to accurately locate
instruction boundaries while disassembling backward. If line info is not
available, the command stops examining memory with an error message.

All the defaults for the arguments to x are designed to make it
easy to continue scanning memory with minimal specifications each time
you use x. For example, after you have inspected three machine
instructions with ‘x/3i addr’, you can inspect the next seven
with just ‘x/7’. If you use RET to repeat the x command,
the repeat count n is used again; the other arguments default as
for successive uses of x.

When examining machine instructions, the instruction at current program
counter is shown with a => marker. For example:

The addresses and contents printed by the x command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
$_ and $__. After an x command, the last address
examined is available for use in expressions in the convenience variable
$_. The contents of that address, as examined, are available in
the convenience variable $__.

If the x command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of output.

Most targets have an addressable memory unit size of 8 bits. This means
that to each memory address are associated 8 bits of data. Some
targets, however, have other addressable memory unit sizes.
Within GDB and this document, the term
addressable memory unit (or memory unit for short) is used
when explicitly referring to a chunk of data of that size. The word
byte is used to refer to a chunk of data of 8 bits, regardless of
the addressable memory unit size of the target. For most systems,
addressable memory unit is a synonym of byte.

When you are debugging a program running on a remote target machine
(see Remote Debugging), you may wish to verify the program’s image
in the remote machine’s memory against the executable file you
downloaded to the target. Or, on any target, you may want to check
whether the program has corrupted its own read-only sections. The
compare-sections command is provided for such situations.

compare-sections [section-name|-r]

Compare the data of a loadable section section-name in the
executable file of the program being debugged with the same section in
the target machine’s memory, and report any mismatches. With no
arguments, compares all loadable sections. With an argument of
-r, compares all loadable read-only sections.

Note: for remote targets, this command can be accelerated if the
target supports computing the CRC checksum of a block of memory
(see qCRC packet).

10.7 Automatic Display

If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the automatic
display list so that GDB prints its value each time your program stops.
Each expression added to the list is given a number to identify it;
to remove an expression from the list, you specify that number.
The automatic display looks like this:

2: foo = 38
3: bar[5] = (struct hack *) 0x3804

This display shows item numbers, expressions and their current values. As with
displays you request manually using x or print, you can
specify the output format you prefer; in fact, display decides
whether to use print or x depending your format
specification—it uses x if you specify either the ‘i’
or ‘s’ format, or a unit size; otherwise it uses print.

display expr

Add the expression expr to the list of expressions to display
each time your program stops. See Expressions.

display does not repeat if you press RET again after using it.

display/fmtexpr

For fmt specifying only a display format and not a size or
count, add the expression expr to the auto-display list but
arrange to display it each time in the specified format fmt.
See Output Formats.

display/fmtaddr

For fmt ‘i’ or ‘s’, or including a unit-size or a
number of units, add the expression addr as a memory address to
be examined each time your program stops. Examining means in effect
doing ‘x/fmtaddr’. See Examining Memory.

For example, ‘display/i $pc’ can be helpful, to see the machine
instruction about to be executed each time execution stops (‘$pc’
is a common name for the program counter; see Registers).

undisplay dnums…

delete display dnums…

Remove items from the list of expressions to display. Specify the
numbers of the displays that you want affected with the command
argument dnums. It can be a single display number, one of the
numbers shown in the first field of the ‘info display’ display;
or it could be a range of display numbers, as in 2-4.

undisplay does not repeat if you press RET after using it.
(Otherwise you would just get the error ‘No display number …’.)

disable display dnums…

Disable the display of item numbers dnums. A disabled display
item is not printed automatically, but is not forgotten. It may be
enabled again later. Specify the numbers of the displays that you
want affected with the command argument dnums. It can be a
single display number, one of the numbers shown in the first field of
the ‘info display’ display; or it could be a range of display
numbers, as in 2-4.

enable display dnums…

Enable display of item numbers dnums. It becomes effective once
again in auto display of its expression, until you specify otherwise.
Specify the numbers of the displays that you want affected with the
command argument dnums. It can be a single display number, one
of the numbers shown in the first field of the ‘info display’
display; or it could be a range of display numbers, as in 2-4.

display

Display the current values of the expressions on the list, just as is
done when your program stops.

info display

Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing the
values. This includes disabled expressions, which are marked as such.
It also includes expressions which would not be displayed right now
because they refer to automatic variables not currently available.

If a display expression refers to local variables, then it does not make
sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command
display last_char while inside a function with an argument
last_char, GDB displays this argument while your program
continues to stop inside that function. When it stops elsewhere—where
there is no variable last_char—the display is disabled
automatically. The next time your program stops where last_char
is meaningful, you can enable the display expression once again.

10.8 Print Settings

GDB provides the following ways to control how arrays, structures,
and symbols are printed.

These settings are useful for debugging programs in any language:

set print address

set print address on

GDB prints memory addresses showing the location of stack
traces, structure values, pointer values, breakpoints, and so forth,
even when it also displays the contents of those addresses. The default
is on. For example, this is what a stack frame display looks like with
set print address on:

You can use ‘set print address off’ to eliminate all machine
dependent displays from the GDB interface. For example, with
print address off, you should get the same text for backtraces on
all machines—whether or not they involve pointer arguments.

show print address

Show whether or not addresses are to be printed.

When GDB prints a symbolic address, it normally prints the
closest earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with
info line, for example ‘info line *0x4537’. Alternately,
you can set GDB to print the source file and line number when
it prints a symbolic address:

set print symbol-filename on

Tell GDB to print the source file name and line number of a
symbol in the symbolic form of an address.

set print symbol-filename off

Do not print source file name and line number of a symbol. This is the
default.

show print symbol-filename

Show whether or not GDB will print the source file name and
line number of a symbol in the symbolic form of an address.

Another situation where it is helpful to show symbol filenames and line
numbers is when disassembling code; GDB shows you the line
number and source file that corresponds to each instruction.

Also, you may wish to see the symbolic form only if the address being
printed is reasonably close to the closest earlier symbol:

set print max-symbolic-offset max-offset

set print max-symbolic-offset unlimited

Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less than
max-offset. The default is unlimited, which tells GDB
to always print the symbolic form of an address if any symbol precedes
it. Zero is equivalent to unlimited.

show print max-symbolic-offset

Ask how large the maximum offset is that GDB prints in a
symbolic address.

If you have a pointer and you are not sure where it points, try
‘set print symbol-filename on’. Then you can determine the name
and source file location of the variable where it points, using
‘p/a pointer’. This interprets the address in symbolic form.
For example, here GDB shows that a variable ptt points
at another variable t, defined in hi2.c:

Warning: For pointers that point to a local variable, ‘p/a’
does not show the symbol name and filename of the referent, even with
the appropriate set print options turned on.

You can also enable ‘/a’-like formatting all the time using
‘set print symbol on’:

set print symbol on

Tell GDB to print the symbol corresponding to an address, if
one exists.

set print symbol off

Tell GDB not to print the symbol corresponding to an
address. In this mode, GDB will still print the symbol
corresponding to pointers to functions. This is the default.

show print symbol

Show whether GDB will display the symbol corresponding to an
address.

Other settings control how different kinds of objects are printed:

set print array

set print array on

Pretty print arrays. This format is more convenient to read,
but uses more space. The default is off.

set print array off

Return to compressed format for arrays.

show print array

Show whether compressed or pretty format is selected for displaying
arrays.

set print array-indexes

set print array-indexes on

Print the index of each element when displaying arrays. May be more
convenient to locate a given element in the array or quickly find the
index of a given element in that printed array. The default is off.

set print array-indexes off

Stop printing element indexes when displaying arrays.

show print array-indexes

Show whether the index of each element is printed when displaying
arrays.

set print elements number-of-elements

set print elements unlimited

Set a limit on how many elements of an array GDB will print.
If GDB is printing a large array, it stops printing after it has
printed the number of elements set by the set print elements command.
This limit also applies to the display of strings.
When GDB starts, this limit is set to 200.
Setting number-of-elements to unlimited or zero means
that the number of elements to print is unlimited.

show print elements

Display the number of elements of a large array that GDB will print.
If the number is 0, then the printing is unlimited.

set print frame-arguments value

This command allows to control how the values of arguments are printed
when the debugger prints a frame (see Frames). The possible
values are:

all

The values of all arguments are printed.

scalars

Print the value of an argument only if it is a scalar. The value of more
complex arguments such as arrays, structures, unions, etc, is replaced
by …. This is the default. Here is an example where
only scalar arguments are shown:

By default, only scalar arguments are printed. This command can be used
to configure the debugger to print the value of all arguments, regardless
of their type. However, it is often advantageous to not print the value
of more complex parameters. For instance, it reduces the amount of
information printed in each frame, making the backtrace more readable.
Also, it improves performance when displaying Ada frames, because
the computation of large arguments can sometimes be CPU-intensive,
especially in large applications. Setting print frame-arguments
to scalars (the default) or none avoids this computation,
thus speeding up the display of each Ada frame.

show print frame-arguments

Show how the value of arguments should be displayed when printing a frame.

set print raw frame-arguments on

Print frame arguments in raw, non pretty-printed, form.

set print raw frame-arguments off

Print frame arguments in pretty-printed form, if there is a pretty-printer
for the value (see Pretty Printing),
otherwise print the value in raw form.
This is the default.

show print raw frame-arguments

Show whether to print frame arguments in raw form.

set print entry-values value

Set printing of frame argument values at function entry. In some cases
GDB can determine the value of function argument which was passed by
the function caller, even if the value was modified inside the called function
and therefore is different. With optimized code, the current value could be
unavailable, but the entry value may still be known.

The default value is default (see below for its description). Older
GDB behaved as with the setting no. Compilers not supporting
this feature will behave in the default setting the same way as with the
no setting.

This functionality is currently supported only by DWARF 2 debugging format and
the compiler has to produce ‘DW_TAG_call_site’ tags. With
GCC, you need to specify -O -g during compilation, to get
this information.

Print the actual parameter value if it is known and also its value from
function entry point if it is known. If neither is known, print for the actual
value <optimized out>. If not in MI mode (see GDB/MI) and if both
values are known and identical, print the shortened
param=param@entry=VALUE notation.

Always print the actual parameter value. Print also its value from function
entry point, but only if it is known. If not in MI mode (see GDB/MI) and
if both values are known and identical, print the shortened
param=param@entry=VALUE notation.

For analysis messages on possible failures of frame argument values at function
entry resolution see set debug entry-values.

show print entry-values

Show the method being used for printing of frame argument values at function
entry.

set print repeats number-of-repeats

set print repeats unlimited

Set the threshold for suppressing display of repeated array
elements. When the number of consecutive identical elements of an
array exceeds the threshold, GDB prints the string
"<repeats n times>", where n is the number of
identical repetitions, instead of displaying the identical elements
themselves. Setting the threshold to unlimited or zero will
cause all elements to be individually printed. The default threshold
is 10.

show print repeats

Display the current threshold for printing repeated identical
elements.

set print null-stop

Cause GDB to stop printing the characters of an array when the first
NULL is encountered. This is useful when large arrays actually
contain only short strings.
The default is off.

show print null-stop

Show whether GDB stops printing an array on the first
NULL character.

set print pretty on

Cause GDB to print structures in an indented format with one member
per line, like this:

Print using only seven-bit characters; if this option is set,
GDB displays any eight-bit characters (in strings or
character values) using the notation \nnn. This setting is
best if you are working in English (ASCII) and you use the
high-order bit of characters as a marker or “meta” bit.

set print sevenbit-strings off

Print full eight-bit characters. This allows the use of more
international character sets, and is the default.

show print sevenbit-strings

Show whether or not GDB is printing only seven-bit characters.

set print union on

Tell GDB to print unions which are contained in structures
and other unions. This is the default setting.

set print union off

Tell GDB not to print unions which are contained in
structures and other unions. GDB will print "{...}"
instead.

show print union

Ask GDB whether or not it will print unions which are contained in
structures and other unions.

set print union affects programs written in C-like languages
and in Pascal.

These settings are of interest when debugging C++ programs:

set print demangle

set print demangle on

Print C++ names in their source form rather than in the encoded
(“mangled”) form passed to the assembler and linker for type-safe
linkage. The default is on.

show print demangle

Show whether C++ names are printed in mangled or demangled form.

set print asm-demangle

set print asm-demangle on

Print C++ names in their source form rather than their mangled form, even
in assembler code printouts such as instruction disassemblies.
The default is off.

show print asm-demangle

Show whether C++ names in assembly listings are printed in mangled
or demangled form.

set demangle-style style

Choose among several encoding schemes used by different compilers to
represent C++ names. The choices for style are currently:

auto

Allow GDB to choose a decoding style by inspecting your program.
This is the default.

gnu

Decode based on the GNU C++ compiler (g++) encoding algorithm.

hp

Decode based on the HP ANSI C++ (aCC) encoding algorithm.

lucid

Decode based on the Lucid C++ compiler (lcc) encoding algorithm.

arm

Decode using the algorithm in the C++ Annotated Reference Manual.
Warning: this setting alone is not sufficient to allow
debugging cfront-generated executables. GDB would
require further enhancement to permit that.

If you omit style, you will see a list of possible formats.

show demangle-style

Display the encoding style currently in use for decoding C++ symbols.

set print object

set print object on

When displaying a pointer to an object, identify the actual
(derived) type of the object rather than the declared type, using
the virtual function table. Note that the virtual function table is
required—this feature can only work for objects that have run-time
type identification; a single virtual method in the object’s declared
type is sufficient. Note that this setting is also taken into account when
working with variable objects via MI (see GDB/MI).

set print object off

Display only the declared type of objects, without reference to the
virtual function table. This is the default setting.

show print object

Show whether actual, or declared, object types are displayed.

set print static-members

set print static-members on

Print static members when displaying a C++ object. The default is on.

set print static-members off

Do not print static members when displaying a C++ object.

show print static-members

Show whether C++ static members are printed or not.

set print pascal_static-members

set print pascal_static-members on

Print static members when displaying a Pascal object. The default is on.

set print pascal_static-members off

Do not print static members when displaying a Pascal object.

show print pascal_static-members

Show whether Pascal static members are printed or not.

set print vtbl

set print vtbl on

Pretty print C++ virtual function tables. The default is off.
(The vtbl commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC).)

10.9.1 Pretty-Printer Introduction

When GDB prints a value, it first sees if there is a pretty-printer
registered for the value. If there is then GDB invokes the
pretty-printer to print the value. Otherwise the value is printed normally.

Pretty-printers are normally named. This makes them easy to manage.
The ‘info pretty-printer’ command will list all the installed
pretty-printers with their names.
If a pretty-printer can handle multiple data types, then its
subprinters are the printers for the individual data types.
Each such subprinter has its own name.
The format of the name is printer-name;subprinter-name.

Pretty-printers are installed by registering them with GDB.
Typically they are automatically loaded and registered when the corresponding
debug information is loaded, thus making them available without having to
do anything special.

There are three places where a pretty-printer can be registered.

Pretty-printers registered globally are available when debugging
all inferiors.

Pretty-printers registered with a program space are available only
when debugging that program.
See Progspaces In Python, for more details on program spaces in Python.

Pretty-printers registered with an objfile are loaded and unloaded
with the corresponding objfile (e.g., shared library).
See Objfiles In Python, for more details on objfiles in Python.

10.9.3 Pretty-Printer Commands

info pretty-printer [object-regexp [name-regexp]]

Print the list of installed pretty-printers.
This includes disabled pretty-printers, which are marked as such.

object-regexp is a regular expression matching the objects
whose pretty-printers to list.
Objects can be global, the program space’s file
(see Progspaces In Python),
and the object files within that program space (see Objfiles In Python).
See Selecting Pretty-Printers, for details on how GDB
looks up a printer from these three objects.

name-regexp is a regular expression matching the name of the printers
to list.

disable pretty-printer [object-regexp [name-regexp]]

Disable pretty-printers matching object-regexp and name-regexp.
A disabled pretty-printer is not forgotten, it may be enabled again later.

enable pretty-printer [object-regexp [name-regexp]]

Enable pretty-printers matching object-regexp and name-regexp.

Example:

Suppose we have three pretty-printers installed: one from library1.so
named foo that prints objects of type foo, and
another from library2.so named bar that prints two types of objects,
bar1 and bar2.

10.10 Value History

Values printed by the print command are saved in the GDBvalue history. This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the file or symbol-file commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.

The values printed are given history numbers by which you can
refer to them. These are successive integers starting with one.
print shows you the history number assigned to a value by
printing ‘$num = ’ before the value; here num is the
history number.

To refer to any previous value, use ‘$’ followed by the value’s
history number. The way print labels its output is designed to
remind you of this. Just $ refers to the most recent value in
the history, and $$ refers to the value before that.
$$n refers to the nth value from the end; $$2
is the value just prior to $$, $$1 is equivalent to
$$, and $$0 is equivalent to $.

For example, suppose you have just printed a pointer to a structure and
want to see the contents of the structure. It suffices to type

p *$

If you have a chain of structures where the component next points
to the next one, you can print the contents of the next one with this:

p *$.next

You can print successive links in the chain by repeating this
command—which you can do by just typing RET.

Note that the history records values, not expressions. If the value of
x is 4 and you type these commands:

print x
set x=5

then the value recorded in the value history by the print command
remains 4 even though the value of x has changed.

show values

Print the last ten values in the value history, with their item numbers.
This is like ‘p $$9’ repeated ten times, except that show
values does not change the history.

show values n

Print ten history values centered on history item number n.

show values +

Print ten history values just after the values last printed. If no more
values are available, show values + produces no display.

Pressing RET to repeat show values n has exactly the
same effect as ‘show values +’.

10.11 Convenience Variables

GDB provides convenience variables that you can use within
GDB to hold on to a value and refer to it later. These variables
exist entirely within GDB; they are not part of your program, and
setting a convenience variable has no direct effect on further execution
of your program. That is why you can use them freely.

Convenience variables are prefixed with ‘$’. Any name preceded by
‘$’ can be used for a convenience variable, unless it is one of
the predefined machine-specific register names (see Registers).
(Value history references, in contrast, are numbers preceded
by ‘$’. See Value History.)

You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program.
For example:

set $foo = *object_ptr

would save in $foo the value contained in the object pointed to by
object_ptr.

Using a convenience variable for the first time creates it, but its
value is void until you assign a new value. You can alter the
value with another assignment at any time.

Convenience variables have no fixed types. You can assign a convenience
variable any type of value, including structures and arrays, even if
that variable already has a value of a different type. The convenience
variable, when used as an expression, has the type of its current value.

show convenience

Print a list of convenience variables used so far, and their values,
as well as a list of the convenience functions.
Abbreviated show conv.

init-if-undefined $variable = expression

Set a convenience variable if it has not already been set. This is useful
for user-defined commands that keep some state. It is similar, in concept,
to using local static variables with initializers in C (except that
convenience variables are global). It can also be used to allow users to
override default values used in a command script.

If the variable is already defined then the expression is not evaluated so
any side-effects do not occur.

One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced. For example, to print
a field from successive elements of an array of structures:

set $i = 0
print bar[$i++]->contents

Repeat that command by typing RET.

Some convenience variables are created automatically by GDB and given
values likely to be useful.

$_

The variable $_ is automatically set by the x command to
the last address examined (see Examining Memory). Other
commands which provide a default address for x to examine also
set $_ to that address; these commands include info line
and info breakpoint. The type of $_ is void *
except when set by the x command, in which case it is a pointer
to the type of $__.

$__

The variable $__ is automatically set by the x command
to the value found in the last address examined. Its type is chosen
to match the format in which the data was printed.

$_exitcode

When the program being debugged terminates normally, GDB
automatically sets this variable to the exit code of the program, and
resets $_exitsignal to void.

$_exitsignal

When the program being debugged dies due to an uncaught signal,
GDB automatically sets this variable to that signal’s number,
and resets $_exitcode to void.

To distinguish between whether the program being debugged has exited
(i.e., $_exitcode is not void) or signalled (i.e.,
$_exitsignal is not void), the convenience function
$_isvoid can be used (see Convenience
Functions). For example, considering the following source code:

A valid way of telling whether the program being debugged has exited
or signalled would be:

(gdb) define has_exited_or_signalled
Type commands for definition of ``has_exited_or_signalled''.
End with a line saying just ``end''.
>if $_isvoid ($_exitsignal)
>echo The program has exited\n
>else
>echo The program has signalled\n
>end
>end
(gdb) run
Starting program:
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists.
(gdb) has_exited_or_signalled
The program has signalled

As can be seen, GDB correctly informs that the program being
debugged has signalled, since it calls raise and raises a
SIGALRM signal. If the program being debugged had not called
raise, then GDB would report a normal exit:

(gdb) has_exited_or_signalled
The program has exited

$_exception

The variable $_exception is set to the exception object being
thrown at an exception-related catchpoint. See Set Catchpoints.

The variable $_sdata contains extra collected static tracepoint
data. See Tracepoint Action Lists. Note that
$_sdata could be empty, if not inspecting a trace buffer, or
if extra static tracepoint data has not been collected.

$_siginfo

The variable $_siginfo contains extra signal information
(see extra signal information). Note that $_siginfo
could be empty, if the application has not yet received any signals.
For example, it will be empty before you execute the run command.

$_tlb

The variable $_tlb is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the qGetTIBAddr request.
See General Query Packets.
This variable contains the address of the thread information block.

10.12 Convenience Functions

GDB also supplies some convenience functions. These
have a syntax similar to convenience variables. A convenience
function can be used in an expression just like an ordinary function;
however, a convenience function is implemented internally to
GDB.

These functions do not require GDB to be configured with
Python support, which means that they are always available.

$_isvoid (expr)

Return one if the expression expr is void. Otherwise it
returns zero.

A void expression is an expression where the type of the result
is void. For example, you can examine a convenience variable
(see Convenience Variables) to check whether
it is void:

In the example above, we used $_isvoid to check whether
$_exitcode is void before and after the execution of the
program being debugged. Before the execution there is no exit code to
be examined, therefore $_exitcode is void. After the
execution the program being debugged returned zero, therefore
$_exitcode is zero, which means that it is not void
anymore.

The void expression can also be a call of a function from the
program being debugged. For example, given the following function:

Returns one if the calling function’s name matches the regular expression
regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided,
it is the number of frames up in the stack to look.
The default is 1.

$_any_caller_is(name[, number_of_frames])

Returns one if any calling function’s name is equal to name.
Otherwise it returns zero.

If the optional argument number_of_frames is provided,
it is the number of frames up in the stack to look.
The default is 1.

This function differs from $_caller_is in that this function
checks all stack frames from the immediate caller to the frame specified
by number_of_frames, whereas $_caller_is only checks the
frame specified by number_of_frames.

$_any_caller_matches(regexp[, number_of_frames])

Returns one if any calling function’s name matches the regular expression
regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided,
it is the number of frames up in the stack to look.
The default is 1.

This function differs from $_caller_matches in that this function
checks all stack frames from the immediate caller to the frame specified
by number_of_frames, whereas $_caller_matches only checks the
frame specified by number_of_frames.

$_as_string(value)

Return the string representation of value.

This function is useful to obtain the textual label (enumerator) of an
enumeration value. For example, assuming the variable node is of
an enumerated type:

10.13 Registers

You can refer to machine register contents, in expressions, as variables
with names starting with ‘$’. The names of registers are different
for each machine; use info registers to see the names used on
your machine.

info registers

Print the names and values of all registers except floating-point
and vector registers (in the selected stack frame).

info all-registers

Print the names and values of all registers, including floating-point
and vector registers (in the selected stack frame).

info registers reggroup …

Print the name and value of the registers in each of the specified
reggroups. The reggoup can be any of those returned by
maint print reggroups (see Maintenance Commands).

info registers regname …

Print the relativized value of each specified register regname.
As discussed in detail below, register values are normally relative to
the selected stack frame. The regname may be any register name valid on
the machine you are using, with or without the initial ‘$’.

GDB has four “standard” register names that are available (in
expressions) on most machines—whenever they do not conflict with an
architecture’s canonical mnemonics for registers. The register names
$pc and $sp are used for the program counter register and
the stack pointer. $fp is used for a register that contains a
pointer to the current stack frame, and $ps is used for a
register that contains the processor status. For example,
you could print the program counter in hex with

Whenever possible, these four standard register names are available on
your machine even though the machine has different canonical mnemonics,
so long as there is no conflict. The info registers command
shows the canonical names. For example, on the SPARC, info
registers displays the processor status register as $psr but you
can also refer to it as $ps; and on x86-based machines $ps
is an alias for the EFLAGS register.

GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way. Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value
(although you can print it as a floating point value with
‘print/f $regname’).

Some registers have distinct “raw” and “virtual” data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in “extended” (raw) format, but all C
programs expect to work with “double” (virtual) format. In such
cases, GDB normally works with the virtual format only (the format
that makes sense for your program), but the info registers command
prints the data in both formats.

Some machines have special registers whose contents can be interpreted
in several different ways. For example, modern x86-based machines
have SSE and MMX registers that can hold several values packed
together in several different formats. GDB refers to such
registers in struct notation:

To set values of such registers, you need to tell GDB which
view of the register you wish to change, as if you were assigning
value to a struct member:

(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF

Normally, register values are relative to the selected stack frame
(see Selecting a Frame). This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost
frame (with ‘frame 0’).

Usually ABIs reserve some registers as not needed to be saved by the
callee (a.k.a.: “caller-saved”, “call-clobbered” or “volatile”
registers). It may therefore not be possible for GDB to know
the value a register had before the call (in other words, in the outer
frame), if the register value has since been changed by the callee.
GDB tries to deduce where the inner frame saved
(“callee-saved”) registers, from the debug info, unwind info, or the
machine code generated by your compiler. If some register is not
saved, and GDB knows the register is “caller-saved” (via
its own knowledge of the ABI, or because the debug/unwind info
explicitly says the register’s value is undefined), GDB
displays ‘<not saved>’ as the register’s value. With targets
that GDB has no knowledge of the register saving convention,
if a register was not saved by the callee, then its value and location
in the outer frame are assumed to be the same of the inner frame.
This is usually harmless, because if the register is call-clobbered,
the caller either does not care what is in the register after the
call, or has code to restore the value that it does care about. Note,
however, that if you change such a register in the outer frame, you
may also be affecting the inner frame. Also, the more “outer” the
frame is you’re looking at, the more likely a call-clobbered
register’s value is to be wrong, in the sense that it doesn’t actually
represent the value the register had just before the call.

10.14 Floating Point Hardware

Depending on the configuration, GDB may be able to give
you more information about the status of the floating point hardware.

info float

Display hardware-dependent information about the floating
point unit. The exact contents and layout vary depending on the
floating point chip. Currently, ‘info float’ is supported on
the ARM and x86 machines.

10.16 Operating System Auxiliary Information

GDB provides interfaces to useful OS facilities that can help
you debug your program.

Some operating systems supply an auxiliary vector to programs at
startup. This is akin to the arguments and environment that you
specify for a program, but contains a system-dependent variety of
binary values that tell system libraries important details about the
hardware, operating system, and process. Each value’s purpose is
identified by an integer tag; the meanings are well-known but system-specific.
Depending on the configuration and operating system facilities,
GDB may be able to show you this information. For remote
targets, this functionality may further depend on the remote stub’s
support of the ‘qXfer:auxv:read’ packet, see
qXfer auxiliary vector read.

info auxv

Display the auxiliary vector of the inferior, which can be either a
live process or a core dump file. GDB prints each tag value
numerically, and also shows names and text descriptions for recognized
tags. Some values in the vector are numbers, some bit masks, and some
pointers to strings or other data. GDB displays each value in the
most appropriate form for a recognized tag, and in hexadecimal for
an unrecognized tag.

On some targets, GDB can access operating system-specific
information and show it to you. The types of information available
will differ depending on the type of operating system running on the
target. The mechanism used to fetch the data is described in
Operating System Information. For remote targets, this
functionality depends on the remote stub’s support of the
‘qXfer:osdata:read’ packet, see qXfer osdata read.

info os infotype

Display OS information of the requested type.

On GNU/Linux, the following values of infotype are valid:

cpus

Display the list of all CPUs/cores. For each CPU/core, GDB prints
the available fields from /proc/cpuinfo. For each supported architecture
different fields are available. Two common entries are processor which gives
CPU number and bogomips; a system constant that is calculated during
kernel initialization.

files

Display the list of open file descriptors on the target. For each
file descriptor, GDB prints the identifier of the process
owning the descriptor, the command of the owning process, the value
of the descriptor, and the target of the descriptor.

modules

Display the list of all loaded kernel modules on the target. For each
module, GDB prints the module name, the size of the module in
bytes, the number of times the module is used, the dependencies of the
module, the status of the module, and the address of the loaded module
in memory.

msg

Display the list of all System V message queues on the target. For each
message queue, GDB prints the message queue key, the message
queue identifier, the access permissions, the current number of bytes
on the queue, the current number of messages on the queue, the processes
that last sent and received a message on the queue, the user and group
of the owner and creator of the message queue, the times at which a
message was last sent and received on the queue, and the time at which
the message queue was last changed.

processes

Display the list of processes on the target. For each process,
GDB prints the process identifier, the name of the user, the
command corresponding to the process, and the list of processor cores
that the process is currently running on. (To understand what these
properties mean, for this and the following info types, please consult
the general GNU/Linux documentation.)

procgroups

Display the list of process groups on the target. For each process,
GDB prints the identifier of the process group that it belongs
to, the command corresponding to the process group leader, the process
identifier, and the command line of the process. The list is sorted
first by the process group identifier, then by the process identifier,
so that processes belonging to the same process group are grouped together
and the process group leader is listed first.

semaphores

Display the list of all System V semaphore sets on the target. For each
semaphore set, GDB prints the semaphore set key, the semaphore
set identifier, the access permissions, the number of semaphores in the
set, the user and group of the owner and creator of the semaphore set,
and the times at which the semaphore set was operated upon and changed.

shm

Display the list of all System V shared-memory regions on the target.
For each shared-memory region, GDB prints the region key,
the shared-memory identifier, the access permissions, the size of the
region, the process that created the region, the process that last
attached to or detached from the region, the current number of live
attaches to the region, and the times at which the region was last
attached to, detach from, and changed.

sockets

Display the list of Internet-domain sockets on the target. For each
socket, GDB prints the address and port of the local and
remote endpoints, the current state of the connection, the creator of
the socket, the IP address family of the socket, and the type of the
connection.

threads

Display the list of threads running on the target. For each thread,
GDB prints the identifier of the process that the thread
belongs to, the command of the process, the thread identifier, and the
processor core that it is currently running on. The main thread of a
process is not listed.

info os

If infotype is omitted, then list the possible values for
infotype and the kind of OS information available for each
infotype. If the target does not return a list of possible
types, this command will report an error.

10.17 Memory Region Attributes

Memory region attributes allow you to describe special handling
required by regions of your target’s memory. GDB uses
attributes to determine whether to allow certain types of memory
accesses; whether to use specific width accesses; and whether to cache
target memory. By default the description of memory regions is
fetched from the target (if the current target supports this), but the
user can override the fetched regions.

Defined memory regions can be individually enabled and disabled. When a
memory region is disabled, GDB uses the default attributes when
accessing memory in that region. Similarly, if no memory regions have
been defined, GDB uses the default attributes when accessing
all memory.

When a memory region is defined, it is given a number to identify it;
to enable, disable, or remove a memory region, you specify that number.

mem lowerupperattributes…

Define a memory region bounded by lower and upper with
attributes attributes…, and add it to the list of regions
monitored by GDB. Note that upper == 0 is a special
case: it is treated as the target’s maximum memory address.
(0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)

mem auto

Discard any user changes to the memory regions and use target-supplied
regions, if available, or no regions if the target does not support.

delete mem nums…

Remove memory regions nums… from the list of regions
monitored by GDB.

disable mem nums…

Disable monitoring of memory regions nums….
A disabled memory region is not forgotten.
It may be enabled again later.

enable mem nums…

Enable monitoring of memory regions nums….

info mem

Print a table of all defined memory regions, with the following columns
for each region:

10.17.1 Attributes

10.17.1.1 Memory Access Mode

The access mode attributes set whether GDB may make read or
write accesses to a memory region.

While these attributes prevent GDB from performing invalid
memory accesses, they do nothing to prevent the target system, I/O DMA,
etc. from accessing memory.

ro

Memory is read only.

wo

Memory is write only.

rw

Memory is read/write. This is the default.

10.17.1.2 Memory Access Size

The access size attribute tells GDB to use specific sized
accesses in the memory region. Often memory mapped device registers
require specific sized accesses. If no access size attribute is
specified, GDB may use accesses of any size.

8

Use 8 bit memory accesses.

16

Use 16 bit memory accesses.

32

Use 32 bit memory accesses.

64

Use 64 bit memory accesses.

10.17.1.3 Data Cache

The data cache attributes set whether GDB will cache target
memory. While this generally improves performance by reducing debug
protocol overhead, it can lead to incorrect results because GDB
does not know about volatile variables or memory mapped device
registers.

cache

Enable GDB to cache target memory.

nocache

Disable GDB from caching target memory. This is the default.

10.17.2 Memory Access Checking

GDB can be instructed to refuse accesses to memory that is
not explicitly described. This can be useful if accessing such
regions has undesired effects for a specific target, or to provide
better error checking. The following commands control this behaviour.

set mem inaccessible-by-default [on|off]

If on is specified, make GDB treat memory not
explicitly described by the memory ranges as non-existent and refuse accesses
to such memory. The checks are only performed if there’s at least one
memory range defined. If off is specified, make GDB
treat the memory not explicitly described by the memory ranges as RAM.
The default value is on.

10.18 Copy Between Memory and a File

You can use the commands dump, append, and
restore to copy data between target memory and a file. The
dump and append commands write data to a file, and the
restore command reads data from a file back into the inferior’s
memory. Files may be in binary, Motorola S-record, Intel hex,
Tektronix Hex, or Verilog Hex format; however, GDB can only
append to binary files, and cannot read from Verilog Hex files.

dump [format] memory filenamestart_addrend_addr

dump [format] value filenameexpr

Dump the contents of memory from start_addr to end_addr,
or the value of expr, to filename in the given format.

The format parameter may be any one of:

binary

Raw binary form.

ihex

Intel hex format.

srec

Motorola S-record format.

tekhex

Tektronix Hex format.

verilog

Verilog Hex format.

GDB uses the same definitions of these formats as the
GNU binary utilities, like ‘objdump’ and ‘objcopy’. If
format is omitted, GDB dumps the data in raw binary
form.

append [binary] memory filenamestart_addrend_addr

append [binary] value filenameexpr

Append the contents of memory from start_addr to end_addr,
or the value of expr, to the file filename, in raw binary form.
(GDB can only append data to files in raw binary form.)

restore filename[binary]biasstartend

Restore the contents of file filename into memory. The
restore command can automatically recognize any known BFD
file format, except for raw binary. To restore a raw binary file you
must specify the optional keyword binary after the filename.

If bias is non-zero, its value will be added to the addresses
contained in the file. Binary files always start at address zero, so
they will be restored at address bias. Other bfd files have
a built-in location; they will be restored at offset bias
from that location.

If start and/or end are non-zero, then only data between
file offset start and file offset end will be restored.
These offsets are relative to the addresses in the file, before
the bias argument is applied.

10.19 How to Produce a Core File from Your Program

A core file or core dump is a file that records the memory
image of a running process and its process status (register values
etc.). Its primary use is post-mortem debugging of a program that
crashed while it ran outside a debugger. A program that crashes
automatically produces a core file, unless this feature is disabled by
the user. See Files, for information on invoking GDB in
the post-mortem debugging mode.

Occasionally, you may wish to produce a core file of the program you
are debugging in order to preserve a snapshot of its state.
GDB has a special command for that.

generate-core-file [file]

gcore [file]

Produce a core dump of the inferior process. The optional argument
file specifies the file name where to put the core dump. If not
specified, the file name defaults to core.pid, where
pid is the inferior process ID.

Note that this command is implemented only for some systems (as of
this writing, GNU/Linux, FreeBSD, Solaris, and S390).

On GNU/Linux, this command can take into account the value of the
file /proc/pid/coredump_filter when generating the core
dump (see set use-coredump-filter), and by default honors the
VM_DONTDUMP flag for mappings where it is present in the file
/proc/pid/smaps (see set dump-excluded-mappings).

set use-coredump-filter on

set use-coredump-filter off

Enable or disable the use of the file
/proc/pid/coredump_filter when generating core dump
files. This file is used by the Linux kernel to decide what types of
memory mappings will be dumped or ignored when generating a core dump
file. pid is the process ID of a currently running process.

To make use of this feature, you have to write in the
/proc/pid/coredump_filter file a value, in hexadecimal,
which is a bit mask representing the memory mapping types. If a bit
is set in the bit mask, then the memory mappings of the corresponding
types will be dumped; otherwise, they will be ignored. This
configuration is inherited by child processes. For more information
about the bits that can be set in the
/proc/pid/coredump_filter file, please refer to the
manpage of core(5).

By default, this option is on. If this option is turned
off, GDB does not read the coredump_filter file
and instead uses the same default value as the Linux kernel in order
to decide which pages will be dumped in the core dump file. This
value is currently 0x33, which means that bits 0
(anonymous private mappings), 1 (anonymous shared mappings),
4 (ELF headers) and 5 (private huge pages) are active.
This will cause these memory mappings to be dumped automatically.

set dump-excluded-mappings on

set dump-excluded-mappings off

If on is specified, GDB will dump memory mappings
marked with the VM_DONTDUMP flag. This flag is represented in
the file /proc/pid/smaps with the acronym dd.

10.20 Character Sets

If the program you are debugging uses a different character set to
represent characters and strings than the one GDB uses itself,
GDB can automatically translate between the character sets for
you. The character set GDB uses we call the host
character set; the one the inferior program uses we call the
target character set.

For example, if you are running GDB on a GNU/Linux system, which
uses the ISO Latin 1 character set, but you are using GDB’s
remote protocol (see Remote Debugging) to debug a program
running on an IBM mainframe, which uses the EBCDIC character set,
then the host character set is Latin-1, and the target character set is
EBCDIC. If you give GDB the command set
target-charset EBCDIC-US, then GDB translates between
EBCDIC and Latin 1 as you print character or string values, or use
character and string literals in expressions.

GDB has no way to automatically recognize which character set
the inferior program uses; you must tell it, using the set
target-charset command, described below.

Here are the commands for controlling GDB’s character set
support:

set target-charset charset

Set the current target character set to charset. To display the
list of supported target character sets, type
set target-charsetTABTAB.

set host-charset charset

Set the current host character set to charset.

By default, GDB uses a host character set appropriate to the
system it is running on; you can override that default using the
set host-charset command. On some systems, GDB cannot
automatically determine the appropriate host character set. In this
case, GDB uses ‘UTF-8’.

GDB can only use certain character sets as its host character
set. If you type set host-charsetTABTAB,
GDB will list the host character sets it supports.

set charset charset

Set the current host and target character sets to charset. As
above, if you type set charset TABTAB,
GDB will list the names of the character sets that can be used
for both host and target.

show charset

Show the names of the current host and target character sets.

show host-charset

Show the name of the current host character set.

show target-charset

Show the name of the current target character set.

set target-wide-charset charset

Set the current target’s wide character set to charset. This is
the character set used by the target’s wchar_t type. To
display the list of supported wide character sets, type
set target-wide-charsetTABTAB.

show target-wide-charset

Show the name of the current target’s wide character set.

Here is an example of GDB’s character set support in action.
Assume that the following source code has been placed in the file
charset-test.c:

We can use the show charset command to see what character sets
GDB is currently using to interpret and display characters and
strings:

(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)

For the sake of printing this manual, let’s use ASCII as our
initial character set:

(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)

Let’s assume that ASCII is indeed the correct character set for our
host system — in other words, let’s assume that if GDB prints
characters using the ASCII character set, our terminal will display
them properly. Since our current target character set is also
ASCII, the contents of ascii_hello print legibly:

We can select IBM1047 as our target character set, and examine the
program’s strings again. Now the ASCII string is wrong, but
GDB translates the contents of ibm1047_hello from the
target character set, IBM1047, to the host character set,
ASCII, and they display correctly:

10.21 Caching Data of Targets

GDB caches data exchanged between the debugger and a target.
Each cache is associated with the address space of the inferior.
See Inferiors and Programs, about inferior and address space.
Such caching generally improves performance in remote debugging
(see Remote Debugging), because it reduces the overhead of the
remote protocol by bundling memory reads and writes into large chunks.
Unfortunately, simply caching everything would lead to incorrect results,
since GDB does not necessarily know anything about volatile
values, memory-mapped I/O addresses, etc. Furthermore, in non-stop mode
(see Non-Stop Mode) memory can be changed while a gdb command
is executing.
Therefore, by default, GDB only caches data
known to be on the stack12 or
in the code segment.
Other regions of memory can be explicitly marked as
cacheable; see Memory Region Attributes.

set remotecache on

set remotecache off

This option no longer does anything; it exists for compatibility
with old scripts.

show remotecache

Show the current state of the obsolete remotecache flag.

set stack-cache on

set stack-cache off

Enable or disable caching of stack accesses. When on, use
caching. By default, this option is on.

show stack-cache

Show the current state of data caching for memory accesses.

set code-cache on

set code-cache off

Enable or disable caching of code segment accesses. When on,
use caching. By default, this option is on. This improves
performance of disassembly in remote debugging.

show code-cache

Show the current state of target memory cache for code segment
accesses.

info dcache [line]

Print the information about the performance of data cache of the
current inferior’s address space. The information displayed
includes the dcache width and depth, and for each cache line, its
number, address, and how many times it was referenced. This
command is useful for debugging the data cache operation.

If a line number is specified, the contents of that line will be
printed in hex.

set dcache size size

Set maximum number of entries in dcache (dcache depth above).

set dcache line-size line-size

Set number of bytes each dcache entry caches (dcache width above).
Must be a power of 2.

10.22 Search Memory

Memory can be searched for a particular sequence of bytes with the
find command.

find [/sn]start_addr, +len, val1[, val2, …]

find [/sn]start_addr, end_addr, val1[, val2, …]

Search memory for the sequence of bytes specified by val1, val2,
etc. The search begins at address start_addr and continues for either
len bytes or through to end_addr inclusive.

s and n are optional parameters.
They may be specified in either order, apart or together.

s, search query size

The size of each search query value.

b

bytes

h

halfwords (two bytes)

w

words (four bytes)

g

giant words (eight bytes)

All values are interpreted in the current language.
This means, for example, that if the current source language is C/C++
then searching for the string “hello” includes the trailing ’\0’.
The null terminator can be removed from searching by using casts,
e.g.: ‘{char[5]}"hello"’.

If the value size is not specified, it is taken from the
value’s type in the current language.
This is useful when one wants to specify the search
pattern as a mixture of types.
Note that this means, for example, that in the case of C-like languages
a search for an untyped 0x42 will search for ‘(int) 0x42’
which is typically four bytes.

n, maximum number of finds

The maximum number of matches to print. The default is to print all finds.

You can use strings as search values. Quote them with double-quotes
(").
The string value is copied into the search pattern byte by byte,
regardless of the endianness of the target and the size specification.

The address of each match found is printed as well as a count of the
number of matches found.

The address of the last value found is stored in convenience variable
‘$_’.
A count of the number of matches is stored in ‘$numfound’.

10.23 Value Sizes

Whenever GDB prints a value memory will be allocated within
GDB to hold the contents of the value. It is possible in
some languages with dynamic typing systems, that an invalid program
may indicate a value that is incorrectly large, this in turn may cause
GDB to try and allocate an overly large ammount of memory.

set max-value-size bytes

set max-value-size unlimited

Set the maximum size of memory that GDB will allocate for the
contents of a value to bytes, trying to display a value that
requires more memory than that will result in an error.

Setting this variable does not effect values that have already been
allocated within GDB, only future allocations.

There’s a minimum size that max-value-size can be set to in
order that GDB can still operate correctly, this minimum is
currently 16 bytes.

The limit applies to the results of some subexpressions as well as to
complete expressions. For example, an expression denoting a simple
integer component, such as x.y.z, may fail if the size of
x.y is dynamic and exceeds bytes. On the other hand,
GDB is sometimes clever; the expression A[i], where
A is an array variable with non-constant size, will generally
succeed regardless of the bounds on A, as long as the component
size is less than bytes.

The default value of max-value-size is currently 64k.

show max-value-size

Show the maximum size of memory, in bytes, that GDB will
allocate for the contents of a value.

11 Debugging Optimized Code

Almost all compilers support optimization. With optimization
disabled, the compiler generates assembly code that corresponds
directly to your source code, in a simplistic way. As the compiler
applies more powerful optimizations, the generated assembly code
diverges from your original source code. With help from debugging
information generated by the compiler, GDB can map from
the running program back to constructs from your original source.

GDB is more accurate with optimization disabled. If you
can recompile without optimization, it is easier to follow the
progress of your program during debugging. But, there are many cases
where you may need to debug an optimized version.

When you debug a program compiled with ‘-g -O’, remember that the
optimizer has rearranged your code; the debugger shows you what is
really there. Do not be too surprised when the execution path does not
exactly match your source file! An extreme example: if you define a
variable, but never use it, GDB never sees that
variable—because the compiler optimizes it out of existence.

Some things do not work as well with ‘-g -O’ as with just
‘-g’, particularly on machines with instruction scheduling. If in
doubt, recompile with ‘-g’ alone, and if this fixes the problem,
please report it to us as a bug (including a test case!).
See Variables, for more information about debugging optimized code.

11.1 Inline Functions

Inlining is an optimization that inserts a copy of the function
body directly at each call site, instead of jumping to a shared
routine. GDB displays inlined functions just like
non-inlined functions. They appear in backtraces. You can view their
arguments and local variables, step into them with step, skip
them with next, and escape from them with finish.
You can check whether a function was inlined by using the
info frame command.

For GDB to support inlined functions, the compiler must
record information about inlining in the debug information —
GCC using the DWARF 2 format does this, and several
other compilers do also. GDB only supports inlined functions
when using DWARF 2. Versions of GCC before 4.1
do not emit two required attributes (‘DW_AT_call_file’ and
‘DW_AT_call_line’); GDB does not display inlined
function calls with earlier versions of GCC. It instead
displays the arguments and local variables of inlined functions as
local variables in the caller.

The body of an inlined function is directly included at its call site;
unlike a non-inlined function, there are no instructions devoted to
the call. GDB still pretends that the call site and the
start of the inlined function are different instructions. Stepping to
the call site shows the call site, and then stepping again shows
the first line of the inlined function, even though no additional
instructions are executed.

This makes source-level debugging much clearer; you can see both the
context of the call and then the effect of the call. Only stepping by
a single instruction using stepi or nexti does not do
this; single instruction steps always show the inlined body.

There are some ways that GDB does not pretend that inlined
function calls are the same as normal calls:

Setting breakpoints at the call site of an inlined function may not
work, because the call site does not contain any code. GDB
may incorrectly move the breakpoint to the next line of the enclosing
function, after the call. This limitation will be removed in a future
version of GDB; until then, set a breakpoint on an earlier line
or inside the inlined function instead.

GDB cannot locate the return value of inlined calls after
using the finish command. This is a limitation of compiler-generated
debugging information; after finish, you can step to the next line
and print a variable where your program stored the return value.

11.2 Tail Call Frames

Function B can call function C in its very last statement. In
unoptimized compilation the call of C is immediately followed by return
instruction at the end of B code. Optimizing compiler may replace the
call and return in function B into one jump to function C
instead. Such use of a jump instruction is called tail call.

During execution of function C, there will be no indication in the
function call stack frames that it was tail-called from B. If function
A regularly calls function B which tail-calls function C,
then GDB will see A as the caller of C. However, in
some cases GDB can determine that C was tail-called from
B, and it will then create fictitious call frame for that, with the
return address set up as if B called C normally.

This functionality is currently supported only by DWARF 2 debugging format and
the compiler has to produce ‘DW_TAG_call_site’ tags. With
GCC, you need to specify -O -g during compilation, to get
this information.

The detection of all the possible code path executions can find them ambiguous.
There is no execution history stored (possible Reverse Execution is never
used for this purpose) and the last known caller could have reached the known
callee by multiple different jump sequences. In such case GDB still
tries to show at least all the unambiguous top tail callers and all the
unambiguous bottom tail calees, if any.

set debug entry-values

When set to on, enables printing of analysis messages for both frame argument
values at function entry and tail calls. It will show all the possible valid
tail calls code paths it has considered. It will also print the intersection
of them with the final unambiguous (possibly partial or even empty) code path
result.

show debug entry-values

Show the current state of analysis messages printing for both frame argument
values at function entry and tail calls.

The analysis messages for tail calls can for example show why the virtual tail
call frame for function c has not been recognized (due to the indirect
reference by variable x):

Frames #0 and #2 are real, #1 is a virtual tail call frame.
The code can have possible execution paths main→a→b→c→d→f or
main→a→b→e→f, GDB cannot find which one from the inferior state.

initial: state shows some random possible calling sequence GDB
has found. It then finds another possible calling sequcen - that one is
prefixed by compare:. The non-ambiguous intersection of these two is
printed as the reduced: calling sequence. That one could have many
futher compare: and reduced: statements as long as there remain
any non-ambiguous sequence entries.

For the frame of function b in both cases there are different possible
$pc values (0x4004cc or 0x4004ce), therefore this frame is
also ambigous. The only non-ambiguous frame is the one for function a,
therefore this one is displayed to the user while the ambiguous frames are
omitted.

There can be also reasons why printing of frame argument values at function
entry may fail:

GDB cannot find out from the inferior state if and how many times did
function a call itself (via function b) as these calls would be
tail calls. Such tail calls would modify thue i variable, therefore
GDB cannot be sure the value it knows would be right - GDB
prints <optimized out> instead.

12 C Preprocessor Macros

Some languages, such as C and C++, provide a way to define and invoke
“preprocessor macros” which expand into strings of tokens.
GDB can evaluate expressions containing macro invocations, show
the result of macro expansion, and show a macro’s definition, including
where it was defined.

You may need to compile your program specially to provide GDB
with information about preprocessor macros. Most compilers do not
include macros in their debugging information, even when you compile
with the -g flag. See Compilation.

A program may define a macro at one point, remove that definition later,
and then provide a different definition after that. Thus, at different
points in the program, a macro may have different definitions, or have
no definition at all. If there is a current stack frame, GDB
uses the macros in scope at that frame’s source code line. Otherwise,
GDB uses the macros in scope at the current listing location;
see List.

Whenever GDB evaluates an expression, it always expands any
macro invocations present in the expression. GDB also provides
the following commands for working with macros explicitly.

macro expand expression

macro exp expression

Show the results of expanding all preprocessor macro invocations in
expression. Since GDB simply expands macros, but does
not parse the result, expression need not be a valid expression;
it can be any string of tokens.

macro expand-once expression

macro exp1 expression

(This command is not yet implemented.) Show the results of
expanding those preprocessor macro invocations that appear explicitly in
expression. Macro invocations appearing in that expansion are
left unchanged. This command allows you to see the effect of a
particular macro more clearly, without being confused by further
expansions. Since GDB simply expands macros, but does not
parse the result, expression need not be a valid expression; it
can be any string of tokens.

info macro [-a|-all] [--] macro

Show the current definition or all definitions of the named macro,
and describe the source location or compiler command-line where that
definition was established. The optional double dash is to signify the end of
argument processing and the beginning of macro for non C-like macros where
the macro may begin with a hyphen.

info macros location

Show all macro definitions that are in effect at the location specified
by location, and describe the source location or compiler
command-line where those definitions were established.

macro define macroreplacement-list

macro define macro(arglist) replacement-list

Introduce a definition for a preprocessor macro named macro,
invocations of which are replaced by the tokens given in
replacement-list. The first form of this command defines an
“object-like” macro, which takes no arguments; the second form
defines a “function-like” macro, which takes the arguments given in
arglist.

A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the
macro undef command, described below. The definition overrides
all definitions for macro present in the program being debugged,
as well as any previous user-supplied definition.

macro undef macro

Remove any user-supplied definition for the macro named macro.
This command only affects definitions provided with the macro
define command, described above; it cannot remove definitions present
in the program being debugged.

macro list

List all the macros defined using the macro define command.

Here is a transcript showing the above commands in action. First, we
show our source files:

In the example above, note that macro expand-once expands only
the macro invocation explicit in the original text — the invocation of
ADD — but does not expand the invocation of the macro M,
which was introduced by ADD.

Once the program is running, GDB uses the macro definitions in
force at the source line of the current stack frame:

In addition to source files, macros can be defined on the compilation command
line using the -Dname=value syntax. For macros defined in
such a way, GDB displays the location of their definition as line zero
of the source file submitted to the compiler.

13 Tracepoints

In some applications, it is not feasible for the debugger to interrupt
the program’s execution long enough for the developer to learn
anything helpful about its behavior. If the program’s correctness
depends on its real-time behavior, delays introduced by a debugger
might cause the program to change its behavior drastically, or perhaps
fail, even when the code itself is correct. It is useful to be able
to observe the program’s behavior without interrupting it.

Using GDB’s trace and collect commands, you can
specify locations in the program, called tracepoints, and
arbitrary expressions to evaluate when those tracepoints are reached.
Later, using the tfind command, you can examine the values
those expressions had when the program hit the tracepoints. The
expressions may also denote objects in memory—structures or arrays,
for example—whose values GDB should record; while visiting
a particular tracepoint, you may inspect those objects as if they were
in memory at that moment. However, because GDB records these
values without interacting with you, it can do so quickly and
unobtrusively, hopefully not disturbing the program’s behavior.

The tracepoint facility is currently available only for remote
targets. See Targets. In addition, your remote target must know
how to collect trace data. This functionality is implemented in the
remote stub; however, none of the stubs distributed with GDB
support tracepoints as of this writing. The format of the remote
packets used to implement tracepoints are described in Tracepoint Packets.

It is also possible to get trace data from a file, in a manner reminiscent
of corefiles; you specify the filename, and use tfind to search
through the file. See Trace Files, for more details.

13.1 Commands to Set Tracepoints

Before running such a trace experiment, an arbitrary number of
tracepoints can be set. A tracepoint is actually a special type of
breakpoint (see Set Breaks), so you can manipulate it using
standard breakpoint commands. For instance, as with breakpoints,
tracepoint numbers are successive integers starting from one, and many
of the commands associated with tracepoints take the tracepoint number
as their argument, to identify which tracepoint to work on.

For each tracepoint, you can specify, in advance, some arbitrary set
of data that you want the target to collect in the trace buffer when
it hits that tracepoint. The collected data can include registers,
local variables, or global data. Later, you can use GDB
commands to examine the values these data had at the time the
tracepoint was hit.

Tracepoints do not support every breakpoint feature. Ignore counts on
tracepoints have no effect, and tracepoints cannot run GDB
commands when they are hit. Tracepoints may not be thread-specific
either.

Some targets may support fast tracepoints, which are inserted in
a different way (such as with a jump instead of a trap), that is
faster but possibly restricted in where they may be installed.

Regular and fast tracepoints are dynamic tracing facilities, meaning
that they can be used to insert tracepoints at (almost) any location
in the target. Some targets may also support controlling static
tracepoints from GDB. With static tracing, a set of
instrumentation points, also known as markers, are embedded in
the target program, and can be activated or deactivated by name or
address. These are usually placed at locations which facilitate
investigating what the target is actually doing. GDB’s
support for static tracing includes being able to list instrumentation
points, and attach them with GDB defined high level
tracepoints that expose the whole range of convenience of
GDB’s tracepoints support. Namely, support for collecting
registers values and values of global or local (to the instrumentation
point) variables; tracepoint conditions and trace state variables.
The act of installing a GDB static tracepoint on an
instrumentation point, or marker, is referred to as probing a
static tracepoint marker.

13.1.1 Create and Delete Tracepoints

trace location

The trace command is very similar to the break command.
Its argument location can be any valid location.
See Specify Location. The trace command defines a tracepoint,
which is a point in the target program where the debugger will briefly stop,
collect some data, and then allow the program to continue. Setting a tracepoint
or changing its actions takes effect immediately if the remote stub
supports the ‘InstallInTrace’ feature (see install tracepoint in tracing).
If remote stub doesn’t support the ‘InstallInTrace’ feature, all
these changes don’t take effect until the next tstart
command, and once a trace experiment is running, further changes will
not have any effect until the next trace experiment starts. In addition,
GDB supports pending tracepoints—tracepoints whose
address is not yet resolved. (This is similar to pending breakpoints.)
Pending tracepoints are not downloaded to the target and not installed
until they are resolved. The resolution of pending tracepoints requires
GDB support—when debugging with the remote target, and
GDB disconnects from the remote stub (see disconnected tracing), pending tracepoints can not be resolved (and downloaded to
the remote stub) while GDB is disconnected.

Set a tracepoint with condition cond; evaluate the expression
cond each time the tracepoint is reached, and collect data only
if the value is nonzero—that is, if cond evaluates as true.
See Tracepoint Conditions, for more
information on tracepoint conditions.

ftrace location [ if cond ]

The ftrace command sets a fast tracepoint. For targets that
support them, fast tracepoints will use a more efficient but possibly
less general technique to trigger data collection, such as a jump
instruction instead of a trap, or some sort of hardware support. It
may not be possible to create a fast tracepoint at the desired
location, in which case the command will exit with an explanatory
message.

GDB handles arguments to ftrace exactly as for
trace.

On 32-bit x86-architecture systems, fast tracepoints normally need to
be placed at an instruction that is 5 bytes or longer, but can be
placed at 4-byte instructions if the low 64K of memory of the target
program is available to install trampolines. Some Unix-type systems,
such as GNU/Linux, exclude low addresses from the program’s
address space; but for instance with the Linux kernel it is possible
to let GDB use this area by doing a sysctl command
to set the mmap_min_addr kernel parameter, as in

sudo sysctl -w vm.mmap_min_addr=32768

which sets the low address to 32K, which leaves plenty of room for
trampolines. The minimum address should be set to a page boundary.

strace location [ if cond ]

The strace command sets a static tracepoint. For targets that
support it, setting a static tracepoint probes a static
instrumentation point, or marker, found at location. It may not
be possible to set a static tracepoint at the desired location, in
which case the command will exit with an explanatory message.

GDB handles arguments to strace exactly as for
trace, with the addition that the user can also specify
-m marker as location. This probes the marker
identified by the marker string identifier. This identifier
depends on the static tracepoint backend library your program is
using. You can find all the marker identifiers in the ‘ID’ field
of the info static-tracepoint-markers command output.
See Listing Static Tracepoint
Markers. For example, in the following small program using the UST
tracing engine:

main ()
{
trace_mark(ust, bar33, "str %s", "FOOBAZ");
}

the marker id is composed of joining the first two arguments to the
trace_mark call with a slash, which translates to:

Static tracepoints accept an extra collect action — collect
$_sdata. This collects arbitrary user data passed in the probe point
call to the tracing library. In the UST example above, you’ll see
that the third argument to trace_mark is a printf-like format
string. The user data is then the result of running that formating
string against the following arguments. Note that info
static-tracepoint-markers command output lists that format string in
the ‘Data:’ field.

You can inspect this data when analyzing the trace buffer, by printing
the $_sdata variable like any other variable available to
GDB. See Tracepoint Action Lists.

The convenience variable $tpnum records the tracepoint number
of the most recently set tracepoint.

delete tracepoint [num]

Permanently delete one or more tracepoints. With no argument, the
default is to delete all tracepoints. Note that the regular
delete command can remove tracepoints also.

13.1.2 Enable and Disable Tracepoints

These commands are deprecated; they are equivalent to plain disable and enable.

disable tracepoint [num]

Disable tracepoint num, or all tracepoints if no argument
num is given. A disabled tracepoint will have no effect during
a trace experiment, but it is not forgotten. You can re-enable
a disabled tracepoint using the enable tracepoint command.
If the command is issued during a trace experiment and the debug target
has support for disabling tracepoints during a trace experiment, then the
change will be effective immediately. Otherwise, it will be applied to the
next trace experiment.

enable tracepoint [num]

Enable tracepoint num, or all tracepoints. If this command is
issued during a trace experiment and the debug target supports enabling
tracepoints during a trace experiment, then the enabled tracepoints will
become effective immediately. Otherwise, they will become effective the
next time a trace experiment is run.

13.1.3 Tracepoint Passcounts

passcount [n[num]]

Set the passcount of a tracepoint. The passcount is a way to
automatically stop a trace experiment. If a tracepoint’s passcount is
n, then the trace experiment will be automatically stopped on
the n’th time that tracepoint is hit. If the tracepoint number
num is not specified, the passcount command sets the
passcount of the most recently defined tracepoint. If no passcount is
given, the trace experiment will run until stopped explicitly by the
user.

13.1.4 Tracepoint Conditions

The simplest sort of tracepoint collects data every time your program
reaches a specified place. You can also specify a condition for
a tracepoint. A condition is just a Boolean expression in your
programming language (see Expressions). A
tracepoint with a condition evaluates the expression each time your
program reaches it, and data collection happens only if the condition
is true.

Tracepoint conditions can be specified when a tracepoint is set, by
using ‘if’ in the arguments to the trace command.
See Setting Tracepoints. They can
also be set or changed at any time with the condition command,
just as with breakpoints.

Unlike breakpoint conditions, GDB does not actually evaluate
the conditional expression itself. Instead, GDB encodes the
expression into an agent expression (see Agent Expressions)
suitable for execution on the target, independently of GDB.
Global variables become raw memory locations, locals become stack
accesses, and so forth.

For instance, suppose you have a function that is usually called
frequently, but should not be called after an error has occurred. You
could use the following tracepoint command to collect data about calls
of that function that happen while the error code is propagating
through the program; an unconditional tracepoint could end up
collecting thousands of useless trace frames that you would have to
search through.

13.1.5 Trace State Variables

A trace state variable is a special type of variable that is
created and managed by target-side code. The syntax is the same as
that for GDB’s convenience variables (a string prefixed with “$”),
but they are stored on the target. They must be created explicitly,
using a tvariable command. They are always 64-bit signed
integers.

Trace state variables are remembered by GDB, and downloaded
to the target along with tracepoint information when the trace
experiment starts. There are no intrinsic limits on the number of
trace state variables, beyond memory limitations of the target.

Although trace state variables are managed by the target, you can use
them in print commands and expressions as if they were convenience
variables; GDB will get the current value from the target
while the trace experiment is running. Trace state variables share
the same namespace as other “$” variables, which means that you
cannot have trace state variables with names like $23 or
$pc, nor can you have a trace state variable and a convenience
variable with the same name.

tvariable $name [ = expression ]

The tvariable command creates a new trace state variable named
$name, and optionally gives it an initial value of
expression. The expression is evaluated when this command is
entered; the result will be converted to an integer if possible,
otherwise GDB will report an error. A subsequent
tvariable command specifying the same name does not create a
variable, but instead assigns the supplied initial value to the
existing variable of that name, overwriting any previous initial
value. The default initial value is 0.

info tvariables

List all the trace state variables along with their initial values.
Their current values may also be displayed, if the trace experiment is
currently running.

delete tvariable [ $name … ]

Delete the given trace state variables, or all of them if no arguments
are specified.

13.1.6 Tracepoint Action Lists

actions [num]

This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number num is not
specified, this command sets the actions for the one that was most
recently defined (so that you can define a tracepoint and then say
actions without bothering about its number). You specify the
actions themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just end. So
far, the only defined actions are collect, teval, and
while-stepping.

actions is actually equivalent to commands (see Breakpoint Command Lists), except that only the defined
actions are allowed; any other GDB command is rejected.

To remove all actions from a tracepoint, type ‘actions num’
and follow it immediately with ‘end’.

In the following example, the action list begins with collect
commands indicating the things to be collected when the tracepoint is
hit. Then, in order to single-step and collect additional data
following the tracepoint, a while-stepping command is used,
followed by the list of things to be collected after each step in a
sequence of single steps. The while-stepping command is
terminated by its own separate end command. Lastly, the action
list is terminated by an end command.

Collect values of the given expressions when the tracepoint is hit.
This command accepts a comma-separated list of any valid expressions.
In addition to global, static, or local variables, the following
special arguments are supported:

$regs

Collect all registers.

$args

Collect all function arguments.

$locals

Collect all local variables.

$_ret

Collect the return address. This is helpful if you want to see more
of a backtrace.

Note: The return address location can not always be reliably
determined up front, and the wrong address / registers may end up
collected instead. On some architectures the reliability is higher
for tracepoints at function entry, while on others it’s the opposite.
When this happens, backtracing will stop because the return address is
found unavailable (unless another collect rule happened to match it).

$_probe_argc

Collects the number of arguments from the static probe at which the
tracepoint is located.
See Static Probe Points.

$_probe_argn

n is an integer between 0 and 11. Collects the nth argument
from the static probe at which the tracepoint is located.
See Static Probe Points.

$_sdata

Collect static tracepoint marker specific data. Only available for
static tracepoints. See Tracepoint Action
Lists. On the UST static tracepoints library backend, an
instrumentation point resembles a printf function call. The
tracing library is able to collect user specified data formatted to a
character string using the format provided by the programmer that
instrumented the program. Other backends have similar mechanisms.
Here’s an example of a UST marker call:

In this case, collecting $_sdata collects the string
‘hello $yourname’. When analyzing the trace buffer, you can
inspect ‘$_sdata’ like any other variable available to
GDB.

You can give several consecutive collect commands, each one
with a single argument, or one collect command with several
arguments separated by commas; the effect is the same.

The optional mods changes the usual handling of the arguments.
s requests that pointers to chars be handled as strings, in
particular collecting the contents of the memory being pointed at, up
to the first zero. The upper bound is by default the value of the
print elements variable; if s is followed by a decimal
number, that is the upper bound instead. So for instance
‘collect/s25 mystr’ collects as many as 25 characters at
‘mystr’.

The command info scope (see info scope) is
particularly useful for figuring out what data to collect.

teval expr1, expr2, …

Evaluate the given expressions when the tracepoint is hit. This
command accepts a comma-separated list of expressions. The results
are discarded, so this is mainly useful for assigning values to trace
state variables (see Trace State Variables) without adding those
values to the trace buffer, as would be the case if the collect
action were used.

while-stepping n

Perform n single-step instruction traces after the tracepoint,
collecting new data after each step. The while-stepping
command is followed by the list of what to collect while stepping
(followed by its own end command):

> while-stepping 12
> collect $regs, myglobal
> end
>

Note that $pc is not automatically collected by
while-stepping; you need to explicitly collect that register if
you need it. You may abbreviate while-stepping as ws or
stepping.

set default-collect expr1, expr2, …

This variable is a list of expressions to collect at each tracepoint
hit. It is effectively an additional collect action prepended
to every tracepoint action list. The expressions are parsed
individually for each tracepoint, so for instance a variable named
xyz may be interpreted as a global for one tracepoint, and a
local for another, as appropriate to the tracepoint’s location.

show default-collect

Show the list of expressions that are collected by default at each
tracepoint hit.

13.1.7 Listing Tracepoints

info tracepoints [num…]

Display information about the tracepoint num. If you don’t
specify a tracepoint number, displays information about all the
tracepoints defined so far. The format is similar to that used for
info breakpoints; in fact, info tracepoints is the same
command, simply restricting itself to tracepoints.

A tracepoint’s listing may include additional information specific to
tracing:

13.1.8 Listing Static Tracepoint Markers

Display information about all static tracepoint markers defined in the
program.

For each marker, the following columns are printed:

Count

An incrementing counter, output to help readability. This is not a
stable identifier.

ID

The marker ID, as reported by the target.

Enabled or Disabled

Probed markers are tagged with ‘y’. ‘n’ identifies marks
that are not enabled.

Address

Where the marker is in your program, as a memory address.

What

Where the marker is in the source for your program, as a file and line
number. If the debug information included in the program does not
allow GDB to locate the source of the marker, this column
will be left blank.

In addition, the following information may be printed for each marker:

Data

User data passed to the tracing library by the marker call. In the
UST backend, this is the format string passed as argument to the
marker call.

13.1.9 Starting and Stopping Trace Experiments

tstart

This command starts the trace experiment, and begins collecting data.
It has the side effect of discarding all the data collected in the
trace buffer during the previous trace experiment. If any arguments
are supplied, they are taken as a note and stored with the trace
experiment’s state. The notes may be arbitrary text, and are
especially useful with disconnected tracing in a multi-user context;
the notes can explain what the trace is doing, supply user contact
information, and so forth.

tstop

This command stops the trace experiment. If any arguments are
supplied, they are recorded with the experiment as a note. This is
useful if you are stopping a trace started by someone else, for
instance if the trace is interfering with the system’s behavior and
needs to be stopped quickly.

Note: a trace experiment and data collection may stop
automatically if any tracepoint’s passcount is reached
(see Tracepoint Passcounts), or if the trace buffer becomes full.

tstatus

This command displays the status of the current trace data
collection.

You can choose to continue running the trace experiment even if
GDB disconnects from the target, voluntarily or
involuntarily. For commands such as detach, the debugger will
ask what you want to do with the trace. But for unexpected
terminations (GDB crash, network outage), it would be
unfortunate to lose hard-won trace data, so the variable
disconnected-tracing lets you decide whether the trace should
continue running without GDB.

set disconnected-tracing on

set disconnected-tracing off

Choose whether a tracing run should continue to run if GDB
has disconnected from the target. Note that detach or
quit will ask you directly what to do about a running trace no
matter what this variable’s setting, so the variable is mainly useful
for handling unexpected situations, such as loss of the network.

show disconnected-tracing

Show the current choice for disconnected tracing.

When you reconnect to the target, the trace experiment may or may not
still be running; it might have filled the trace buffer in the
meantime, or stopped for one of the other reasons. If it is running,
it will continue after reconnection.

Upon reconnection, the target will upload information about the
tracepoints in effect. GDB will then compare that
information to the set of tracepoints currently defined, and attempt
to match them up, allowing for the possibility that the numbers may
have changed due to creation and deletion in the meantime. If one of
the target’s tracepoints does not match any in GDB, the
debugger will create a new tracepoint, so that you have a number with
which to specify that tracepoint. This matching-up process is
necessarily heuristic, and it may result in useless tracepoints being
created; you may simply delete them if they are of no use.

If your target agent supports a circular trace buffer, then you
can run a trace experiment indefinitely without filling the trace
buffer; when space runs out, the agent deletes already-collected trace
frames, oldest first, until there is enough room to continue
collecting. This is especially useful if your tracepoints are being
hit too often, and your trace gets terminated prematurely because the
buffer is full. To ask for a circular trace buffer, simply set
‘circular-trace-buffer’ to on. You can set this at any time,
including during tracing; if the agent can do it, it will change
buffer handling on the fly, otherwise it will not take effect until
the next run.

set circular-trace-buffer on

set circular-trace-buffer off

Choose whether a tracing run should use a linear or circular buffer
for trace data. A linear buffer will not lose any trace data, but may
fill up prematurely, while a circular buffer will discard old trace
data, but it will have always room for the latest tracepoint hits.

show circular-trace-buffer

Show the current choice for the trace buffer. Note that this may not
match the agent’s current buffer handling, nor is it guaranteed to
match the setting that might have been in effect during a past run,
for instance if you are looking at frames from a trace file.

set trace-buffer-size n

set trace-buffer-size unlimited

Request that the target use a trace buffer of n bytes. Not all
targets will honor the request; they may have a compiled-in size for
the trace buffer, or some other limitation. Set to a value of
unlimited or -1 to let the target use whatever size it
likes. This is also the default.

show trace-buffer-size

Show the current requested size for the trace buffer. Note that this
will only match the actual size if the target supports size-setting,
and was able to handle the requested size. For instance, if the
target can only change buffer size between runs, this variable will
not reflect the change until the next run starts. Use tstatus
to get a report of the actual buffer size.

set trace-user text

show trace-user

set trace-notes text

Set the trace run’s notes.

show trace-notes

Show the trace run’s notes.

set trace-stop-notes text

Set the trace run’s stop notes. The handling of the note is as for
tstop arguments; the set command is convenient way to fix a
stop note that is mistaken or incomplete.

13.1.10 Tracepoint Restrictions

There are a number of restrictions on the use of tracepoints. As
described above, tracepoint data gathering occurs on the target
without interaction from GDB. Thus the full capabilities of
the debugger are not available during data gathering, and then at data
examination time, you will be limited by only having what was
collected. The following items describe some common problems, but it
is not exhaustive, and you may run into additional difficulties not
mentioned here.

Tracepoint expressions are intended to gather objects (lvalues). Thus
the full flexibility of GDB’s expression evaluator is not available.
You cannot call functions, cast objects to aggregate types, access
convenience variables or modify values (except by assignment to trace
state variables). Some language features may implicitly call
functions (for instance Objective-C fields with accessors), and therefore
cannot be collected either.

Collection of local variables, either individually or in bulk with
$locals or $args, during while-stepping may
behave erratically. The stepping action may enter a new scope (for
instance by stepping into a function), or the location of the variable
may change (for instance it is loaded into a register). The
tracepoint data recorded uses the location information for the
variables that is correct for the tracepoint location. When the
tracepoint is created, it is not possible, in general, to determine
where the steps of a while-stepping sequence will advance the
program—particularly if a conditional branch is stepped.

Collection of an incompletely-initialized or partially-destroyed object
may result in something that GDB cannot display, or displays
in a misleading way.

When GDB displays a pointer to character it automatically
dereferences the pointer to also display characters of the string
being pointed to. However, collecting the pointer during tracing does
not automatically collect the string. You need to explicitly
dereference the pointer and provide size information if you want to
collect not only the pointer, but the memory pointed to. For example,
*ptr@50 can be used to collect the 50 element array pointed to
by ptr.

It is not possible to collect a complete stack backtrace at a
tracepoint. Instead, you may collect the registers and a few hundred
bytes from the stack pointer with something like *(unsigned char *)$esp@300
(adjust to use the name of the actual stack pointer register on your
target architecture, and the amount of stack you wish to capture).
Then the backtrace command will show a partial backtrace when
using a trace frame. The number of stack frames that can be examined
depends on the sizes of the frames in the collected stack. Note that
if you ask for a block so large that it goes past the bottom of the
stack, the target agent may report an error trying to read from an
invalid address.

If you do not collect registers at a tracepoint, GDB can
infer that the value of $pc must be the same as the address of
the tracepoint and use that when you are looking at a trace frame
for that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that was
inlined), or if it has a while-stepping loop. In those cases
GDB will warn you that it can’t infer $pc, and default
it to zero.

13.2 Using the Collected Data

After the tracepoint experiment ends, you use GDB commands
for examining the trace data. The basic idea is that each tracepoint
collects a trace snapshot every time it is hit and another
snapshot every time it single-steps. All these snapshots are
consecutively numbered from zero and go into a buffer, and you can
examine them later. The way you examine them is to focus on a
specific trace snapshot. When the remote stub is focused on a trace
snapshot, it will respond to all GDB requests for memory and
registers by reading from the buffer which belongs to that snapshot,
rather than from real memory or registers of the program being
debugged. This means that allGDB commands
(print, info registers, backtrace, etc.) will
behave as if we were currently debugging the program state as it was
when the tracepoint occurred. Any requests for data that are not in
the buffer will fail.

13.2.1 tfind n

The basic command for selecting a trace snapshot from the buffer is
tfind n, which finds trace snapshot number n,
counting from zero. If no argument n is given, the next
snapshot is selected.

Here are the various forms of using the tfind command.

tfind start

Find the first snapshot in the buffer. This is a synonym for
tfind 0 (since 0 is the number of the first snapshot).

tfind none

Stop debugging trace snapshots, resume live debugging.

tfind end

Same as ‘tfind none’.

tfind

No argument means find the next trace snapshot or find the first
one if no trace snapshot is selected.

tfind -

Find the previous trace snapshot before the current one. This permits
retracing earlier steps.

tfind tracepoint num

Find the next snapshot associated with tracepoint num. Search
proceeds forward from the last examined trace snapshot. If no
argument num is given, it means find the next snapshot collected
for the same tracepoint as the current snapshot.

tfind pc addr

Find the next snapshot associated with the value addr of the
program counter. Search proceeds forward from the last examined trace
snapshot. If no argument addr is given, it means find the next
snapshot with the same value of PC as the current snapshot.

tfind outside addr1, addr2

Find the next snapshot whose PC is outside the given range of
addresses (exclusive).

tfind range addr1, addr2

Find the next snapshot whose PC is between addr1 and
addr2 (inclusive).

tfind line [file:]n

Find the next snapshot associated with the source line n. If
the optional argument file is given, refer to line n in
that source file. Search proceeds forward from the last examined
trace snapshot. If no argument n is given, it means find the
next line other than the one currently being examined; thus saying
tfind line repeatedly can appear to have the same effect as
stepping from line to line in a live debugging session.

The default arguments for the tfind commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, tfind with no argument selects the next trace
snapshot, and tfind - with no argument selects the previous
trace snapshot. So, by giving one tfind command, and then
simply hitting RET repeatedly you can examine all the trace
snapshots in order. Or, by saying tfind - and then hitting
RET repeatedly you can examine the snapshots in reverse order.
The tfind line command with no argument selects the snapshot
for the next source line executed. The tfind pc command with
no argument selects the next snapshot with the same program counter
(PC) as the current frame. The tfind tracepoint command with
no argument selects the next trace snapshot collected by the same
tracepoint as the current one.

In addition to letting you scan through the trace buffer manually,
these commands make it easy to construct GDB scripts that
scan through the trace buffer and print out whatever collected data
you are interested in. Thus, if we want to examine the PC, FP, and SP
registers from each trace frame in the buffer, we can say this:

tdump works by scanning the tracepoint’s current collection
actions and printing the value of each expression listed. So
tdump can fail, if after a run, you change the tracepoint’s
actions to mention variables that were not collected during the run.

Also, for tracepoints with while-stepping loops, tdump
uses the collected value of $pc to distinguish between trace
frames that were collected at the tracepoint hit, and frames that were
collected while stepping. This allows it to correctly choose whether
to display the basic list of collections, or the collections from the
body of the while-stepping loop. However, if $pc was not collected,
then tdump will always attempt to dump using the basic collection
list, and may fail if a while-stepping frame does not include all the
same data that is collected at the tracepoint hit.

13.2.3 save tracepoints filename

This command saves all current tracepoint definitions together with
their actions and passcounts, into a file filename
suitable for use in a later debugging session. To read the saved
tracepoint definitions, use the source command (see Command Files). The save-tracepoints command is a deprecated
alias for save tracepoints

13.3 Convenience Variables for Tracepoints

The current trace snapshot (a.k.a. frame) number, or -1 if no
snapshot is selected.

(int) $tracepoint

The tracepoint for the current trace snapshot.

(int) $trace_line

The line number for the current trace snapshot.

(char []) $trace_file

The source file for the current trace snapshot.

(char []) $trace_func

The name of the function containing $tracepoint.

Note: $trace_file is not suitable for use in printf,
use output instead.

Here’s a simple example of using these convenience variables for
stepping through all the trace snapshots and printing some of their
data. Note that these are not the same as trace state variables,
which are managed by the target.

13.4 Using Trace Files

In some situations, the target running a trace experiment may no
longer be available; perhaps it crashed, or the hardware was needed
for a different activity. To handle these cases, you can arrange to
dump the trace data into a file, and later use that file as a source
of trace data, via the target tfile command.

tsave [ -r ] filename

tsave [-ctf] dirname

Save the trace data to filename. By default, this command
assumes that filename refers to the host filesystem, so if
necessary GDB will copy raw trace data up from the target and
then save it. If the target supports it, you can also supply the
optional argument -r (“remote”) to direct the target to save
the data directly into filename in its own filesystem, which may be
more efficient if the trace buffer is very large. (Note, however, that
target tfile can only read from files accessible to the host.)
By default, this command will save trace frame in tfile format.
You can supply the optional argument -ctf to save data in CTF
format. The Common Trace Format (CTF) is proposed as a trace format
that can be shared by multiple debugging and tracing tools. Please go to
‘http://www.efficios.com/ctf’ to get more information.

target tfile filename

target ctf dirname

Use the file named filename or directory named dirname as
a source of trace data. Commands that examine data work as they do with
a live target, but it is not possible to run any new trace experiments.
tstatus will report the state of the trace run at the moment
the data was saved, as well as the current trace frame you are examining.
Both filename and dirname must be on a filesystem accessible to
the host.

14 Debugging Programs That Use Overlays

If your program is too large to fit completely in your target system’s
memory, you can sometimes use overlays to work around this
problem. GDB provides some support for debugging programs that
use overlays.

14.1 How Overlays Work

Suppose you have a computer whose instruction address space is only 64
kilobytes long, but which has much more memory which can be accessed by
other means: special instructions, segment registers, or memory
management hardware, for example. Suppose further that you want to
adapt a program which is larger than 64 kilobytes to run on this system.

One solution is to identify modules of your program which are relatively
independent, and need not call each other directly; call these modules
overlays. Separate the overlays from the main program, and place
their machine code in the larger memory. Place your main program in
instruction memory, but leave at least enough space there to hold the
largest overlay as well.

Now, to call a function located in an overlay, you must first copy that
overlay’s machine code from the large memory into the space set aside
for it in the instruction memory, and then jump to its entry point
there.

The diagram (see A code overlay) shows a system with separate data
and instruction address spaces. To map an overlay, the program copies
its code from the larger address space to the instruction address space.
Since the overlays shown here all use the same mapped address, only one
may be mapped at a time. For a system with a single address space for
data and instructions, the diagram would be similar, except that the
program variables and heap would share an address space with the main
program and the overlay area.

An overlay loaded into instruction memory and ready for use is called a
mapped overlay; its mapped address is its address in the
instruction memory. An overlay not present (or only partially present)
in instruction memory is called unmapped; its load address
is its address in the larger memory. The mapped address is also called
the virtual memory address, or VMA; the load address is also
called the load memory address, or LMA.

Unfortunately, overlays are not a completely transparent way to adapt a
program to limited instruction memory. They introduce a new set of
global constraints you must keep in mind as you design your program:

Before calling or returning to a function in an overlay, your program
must make sure that overlay is actually mapped. Otherwise, the call or
return will transfer control to the right address, but in the wrong
overlay, and your program will probably crash.

If the process of mapping an overlay is expensive on your system, you
will need to choose your overlays carefully to minimize their effect on
your program’s performance.

The executable file you load onto your system must contain each
overlay’s instructions, appearing at the overlay’s load address, not its
mapped address. However, each overlay’s instructions must be relocated
and its symbols defined as if the overlay were at its mapped address.
You can use GNU linker scripts to specify different load and relocation
addresses for pieces of your program; see Overlay Description in Using ld: the GNU linker.

The procedure for loading executable files onto your system must be able
to load their contents into the larger address space as well as the
instruction and data spaces.

The overlay system described above is rather simple, and could be
improved in many ways:

If your system has suitable bank switch registers or memory management
hardware, you could use those facilities to make an overlay’s load area
contents simply appear at their mapped address in instruction space.
This would probably be faster than copying the overlay to its mapped
area in the usual way.

If your overlays are small enough, you could set aside more than one
overlay area, and have more than one overlay mapped at a time.

You can use overlays to manage data, as well as instructions. In
general, data overlays are even less transparent to your design than
code overlays: whereas code overlays only require care when you call or
return to functions, data overlays require care every time you access
the data. Also, if you change the contents of a data overlay, you
must copy its contents back out to its load address before you can copy a
different data overlay into the same mapped area.

14.2 Overlay Commands

To use GDB’s overlay support, each overlay in your program must
correspond to a separate section of the executable file. The section’s
virtual memory address and load memory address must be the overlay’s
mapped and load addresses. Identifying overlays with sections allows
GDB to determine the appropriate address of a function or
variable, depending on whether the overlay is mapped or not.

GDB’s overlay commands all start with the word overlay;
you can abbreviate this as ov or ovly. The commands are:

overlay off

Disable GDB’s overlay support. When overlay support is
disabled, GDB assumes that all functions and variables are
always present at their mapped addresses. By default, GDB’s
overlay support is disabled.

overlay manual

Enable manual overlay debugging. In this mode, GDB
relies on you to tell it which overlays are mapped, and which are not,
using the overlay map-overlay and overlay unmap-overlay
commands described below.

overlay map-overlay overlay

overlay map overlay

Tell GDB that overlay is now mapped; overlay must
be the name of the object file section containing the overlay. When an
overlay is mapped, GDB assumes it can find the overlay’s
functions and variables at their mapped addresses. GDB assumes
that any other overlays whose mapped ranges overlap that of
overlay are now unmapped.

overlay unmap-overlay overlay

overlay unmap overlay

Tell GDB that overlay is no longer mapped; overlay
must be the name of the object file section containing the overlay.
When an overlay is unmapped, GDB assumes it can find the
overlay’s functions and variables at their load addresses.

overlay auto

Enable automatic overlay debugging. In this mode, GDB
consults a data structure the overlay manager maintains in the inferior
to see which overlays are mapped. For details, see Automatic Overlay Debugging.

overlay load-target

overlay load

Re-read the overlay table from the inferior. Normally, GDB
re-reads the table GDB automatically each time the inferior
stops, so this command should only be necessary if you have changed the
overlay mapping yourself using GDB. This command is only
useful when using automatic overlay debugging.

overlay list-overlays

overlay list

Display a list of the overlays currently mapped, along with their mapped
addresses, load addresses, and sizes.

Normally, when GDB prints a code address, it includes the name
of the function the address falls in:

(gdb) print main
$3 = {int ()} 0x11a0 <main>

When overlay debugging is enabled, GDB recognizes code in
unmapped overlays, and prints the names of unmapped functions with
asterisks around them. For example, if foo is a function in an
unmapped overlay, GDB prints it this way:

When overlay debugging is enabled, GDB can find the correct
address for functions and variables in an overlay, whether or not the
overlay is mapped. This allows most GDB commands, like
break and disassemble, to work normally, even on unmapped
code. However, GDB’s breakpoint support has some limitations:

You can set breakpoints in functions in unmapped overlays, as long as
GDB can write to the overlay at its load address.

GDB can not set hardware or simulator-based breakpoints in
unmapped overlays. However, if you set a breakpoint at the end of your
overlay manager (and tell GDB which overlays are now mapped, if
you are using manual overlay management), GDB will re-set its
breakpoints properly.

14.3 Automatic Overlay Debugging

GDB can automatically track which overlays are mapped and which
are not, given some simple co-operation from the overlay manager in the
inferior. If you enable automatic overlay debugging with the
overlay auto command (see Overlay Commands), GDB
looks in the inferior’s memory for certain variables describing the
current state of the overlays.

Here are the variables your overlay manager must define to support
GDB’s automatic overlay debugging:

This variable must be a four-byte signed integer, holding the total
number of elements in _ovly_table.

To decide whether a particular overlay is mapped or not, GDB
looks for an entry in _ovly_table whose vma and
lma members equal the VMA and LMA of the overlay’s section in the
executable file. When GDB finds a matching entry, it consults
the entry’s mapped member to determine whether the overlay is
currently mapped.

In addition, your overlay manager may define a function called
_ovly_debug_event. If this function is defined, GDB
will silently set a breakpoint there. If the overlay manager then
calls this function whenever it has changed the overlay table, this
will enable GDB to accurately keep track of which overlays
are in program memory, and update any breakpoints that may be set
in overlays. This will allow breakpoints to work even if the
overlays are kept in ROM or other non-writable memory while they
are not being executed.

14.4 Overlay Sample Program

When linking a program which uses overlays, you must place the overlays
at their load addresses, while relocating them to run at their mapped
addresses. To do this, you must write a linker script (see Overlay
Description in Using ld: the GNU linker). Unfortunately,
since linker scripts are specific to a particular host system, target
architecture, and target memory layout, this manual cannot provide
portable sample code demonstrating GDB’s overlay support.

However, the GDB source distribution does contain an overlaid
program, with linker scripts for a few systems, as part of its test
suite. The program consists of the following files from
gdb/testsuite/gdb.base:

overlays.c

The main program file.

ovlymgr.c

A simple overlay manager, used by overlays.c.

foo.c

bar.c

baz.c

grbx.c

Overlay modules, loaded and used by overlays.c.

d10v.ld

m32r.ld

Linker scripts for linking the test program on the d10v-elf
and m32r-elf targets.

You can build the test program using the d10v-elf GCC
cross-compiler like this:

15 Using GDB with Different Languages

Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer p is accomplished by *p, but in
Modula-2, it is accomplished by p^. Values can also be
represented (and displayed) differently. Hex numbers in C appear as
‘0x1ae’, while in Modula-2 they appear as ‘1AEH’.

Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program’s
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program’s native language. The
language you use to build expressions is called the working
language.

15.1 Switching Between Source Languages

There are two ways to control the working language—either have GDB
set it automatically, or select it manually yourself. You can use the
set language command for either purpose. On startup, GDB
defaults to setting the language automatically. The working language is
used to determine how expressions you type are interpreted, how values
are printed, etc.

In addition to the working language, every source file that
GDB knows about has its own working language. For some object
file formats, the compiler might indicate which language a particular
source file is in. However, most of the time GDB infers the
language from the name of the file. The language of a source file
controls whether C++ names are demangled—this way backtrace can
show each frame appropriately for its own language. There is no way to
set the language of a source file from within GDB, but you can
set the language associated with a filename extension. See Displaying the Language.

This is most commonly a problem when you use a program, such
as cfront or f2c, that generates C but is written in
another language. In that case, make the
program use #line directives in its C output; that way
GDB will know the correct language of the source code of the original
program, and will display that source code, not the generated C code.

15.1.2 Setting the Working Language

If you allow GDB to set the language automatically,
expressions are interpreted the same way in your debugging session and
your program.

If you wish, you may set the language manually. To do this, issue the
command ‘set language lang’, where lang is the name of
a language, such as
c or modula-2.
For a list of the supported languages, type ‘set language’.

Setting the language manually prevents GDB from updating the working
language automatically. This can lead to confusion if you try
to debug a program when the working language is not the same as the
source language, when an expression is acceptable to both
languages—but means different things. For instance, if the current
source file were written in C, and GDB was parsing Modula-2, a
command such as:

print a = b + c

might not have the effect you intended. In C, this means to add
b and c and place the result in a. The result
printed would be the value of a. In Modula-2, this means to compare
a to the result of b+c, yielding a BOOLEAN value.

15.1.3 Having GDB Infer the Source Language

To have GDB set the working language automatically, use
‘set language local’ or ‘set language auto’. GDB
then infers the working language. That is, when your program stops in a
frame (usually by encountering a breakpoint), GDB sets the
working language to the language recorded for the function in that
frame. If the language for a frame is unknown (that is, if the function
or block corresponding to the frame was defined in a source file that
does not have a recognized extension), the current working language is
not changed, and GDB issues a warning.

This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using ‘set language auto’ in this
case frees you from having to set the working language manually.

15.2 Displaying the Language

The following commands help you find out which language is the
working language, and also what language source files were written in.

show language

Display the current working language. This is the
language you can use with commands such as print to
build and compute expressions that may involve variables in your program.

info frame

Display the source language for this frame. This language becomes the
working language if you use an identifier from this frame.
See Information about a Frame, to identify the other
information listed here.

15.3 Type and Range Checking

Some languages are designed to guard you against making seemingly common
errors through a series of compile- and run-time checks. These include
checking the type of arguments to functions and operators and making
sure mathematical overflows are caught at run time. Checks such as
these help to ensure a program’s correctness once it has been compiled
by eliminating type mismatches and providing active checks for range
errors when your program is running.

By default GDB checks for these errors according to the
rules of the current source language. Although GDB does not check
the statements in your program, it can check expressions entered directly
into GDB for evaluation via the print command, for example.

15.3.1 An Overview of Type Checking

Some languages, such as C and C++, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs. These checks prevent type mismatch
errors from ever causing any run-time problems. For example,

The second example fails because in C++ the integer constant
‘0x1234’ is not type-compatible with the pointer parameter type.

For the expressions you use in GDB commands, you can tell
GDB to not enforce strict type checking or
to treat any mismatches as errors and abandon the expression;
When type checking is disabled, GDB successfully evaluates
expressions like the second example above.

Even if type checking is off, there may be other reasons
related to type that prevent GDB from evaluating an expression.
For instance, GDB does not know how to add an int and
a struct foo. These particular type errors have nothing to do
with the language in use and usually arise from expressions which make
little sense to evaluate anyway.

GDB provides some additional commands for controlling type checking:

set check type on

set check type off

Set strict type checking on or off. If any type mismatches occur in
evaluating an expression while type checking is on, GDB prints a
message and aborts evaluation of the expression.

show check type

Show the current setting of type checking and whether GDB
is enforcing strict type checking rules.

15.3.2 An Overview of Range Checking

In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks. Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.

For expressions you use in GDB commands, you can tell
GDB to treat range errors in one of three ways: ignore them,
always treat them as errors and abandon the expression, or issue
warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member
of any type. Some languages, however, do not treat overflows as an
error. In many implementations of C, mathematical overflow causes the
result to “wrap around” to lower values—for example, if m is
the largest integer value, and s is the smallest, then

m + 1 ⇒ s

This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines. See Supported Languages, for further details on specific languages.

GDB provides some additional commands for controlling the range checker:

set check range auto

Set range checking on or off based on the current working language.
See Supported Languages, for the default settings for
each language.

set check range on

set check range off

Set range checking on or off, overriding the default setting for the
current working language. A warning is issued if the setting does not
match the language default. If a range error occurs and range checking is on,
then a message is printed and evaluation of the expression is aborted.

set check range warn

Output messages when the GDB range checker detects a range error,
but attempt to evaluate the expression anyway. Evaluating the
expression may still be impossible for other reasons, such as accessing
memory that the process does not own (a typical example from many Unix
systems).

show range

Show the current setting of the range checker, and whether or not it is
being set automatically by GDB.

15.4 Supported Languages

GDB supports C, C++, D, Go, Objective-C, Fortran,
OpenCL C, Pascal, Rust, assembly, Modula-2, and Ada.
Some GDB features may be used in expressions regardless of the
language you use: the GDB@ and :: operators,
and the ‘{type}addr’ construct (see Expressions) can be used with the constructs of any supported
language.

The following sections detail to what degree each source language is
supported by GDB. These sections are not meant to be language
tutorials or references, but serve only as a reference guide to what the
GDB expression parser accepts, and what input and output
formats should look like for different languages. There are many good
books written on each of these languages; please look to these for a
language reference or tutorial.

15.4.1 C and C++

Since C and C++ are so closely related, many features of GDB apply
to both languages. Whenever this is the case, we discuss those languages
together.

The C++ debugging facilities are jointly implemented by the C++
compiler and GDB. Therefore, to debug your C++ code
effectively, you must compile your C++ programs with a supported
C++ compiler, such as GNUg++, or the HP ANSI C++
compiler (aCC).

Multiplication, division, and modulus. Multiplication and division are
defined on integral and floating-point types. Modulus is defined on
integral types.

++, --

Increment and decrement. When appearing before a variable, the
operation is performed before the variable is used in an expression;
when appearing after it, the variable’s value is used before the
operation takes place.

*

Pointer dereferencing. Defined on pointer types. Same precedence as
++.

&

Address operator. Defined on variables. Same precedence as ++.

For debugging C++, GDB implements a use of ‘&’ beyond what is
allowed in the C++ language itself: you can use ‘&(&ref)’
to examine the address
where a C++ reference variable (declared with ‘&ref’) is
stored.

-

Negative. Defined on integral and floating-point types. Same
precedence as ++.

Structure member, and pointer-to-structure member. For convenience,
GDB regards the two as equivalent, choosing whether to dereference a
pointer based on the stored type information.
Defined on struct and union data.

15.4.1.2 C and C++ Constants

GDB allows you to express the constants of C and C++ in the
following ways:

Integer constants are a sequence of digits. Octal constants are
specified by a leading ‘0’ (i.e. zero), and hexadecimal constants
by a leading ‘0x’ or ‘0X’. Constants may also end with a letter
‘l’, specifying that the constant should be treated as a
long value.

Floating point constants are a sequence of digits, followed by a decimal
point, followed by a sequence of digits, and optionally followed by an
exponent. An exponent is of the form:
‘e[[+]|-]nnn’, where nnn is another
sequence of digits. The ‘+’ is optional for positive exponents.
A floating-point constant may also end with a letter ‘f’ or
‘F’, specifying that the constant should be treated as being of
the float (as opposed to the default double) type; or with
a letter ‘l’ or ‘L’, which specifies a long double
constant.

Enumerated constants consist of enumerated identifiers, or their
integral equivalents.

Character constants are a single character surrounded by single quotes
('), or a number—the ordinal value of the corresponding character
(usually its ASCII value). Within quotes, the single character may
be represented by a letter or by escape sequences, which are of
the form ‘\nnn’, where nnn is the octal representation
of the character’s ordinal value; or of the form ‘\x’, where
‘x’ is a predefined special character—for example,
‘\n’ for newline.

Wide character constants can be written by prefixing a character
constant with ‘L’, as in C. For example, ‘L'x'’ is the wide
form of ‘x’. The target wide character set is used when
computing the value of this constant (see Character Sets).

String constants are a sequence of character constants surrounded by
double quotes ("). Any valid character constant (as described
above) may appear. Double quotes within the string must be preceded by
a backslash, so for instance ‘"a\"b'c"’ is a string of five
characters.

Wide string constants can be written by prefixing a string constant
with ‘L’, as in C. The target wide character set is used when
computing the value of this constant (see Character Sets).

Pointer constants are an integral value. You can also write pointers
to constants using the C operator ‘&’.

Array constants are comma-separated lists surrounded by braces ‘{’
and ‘}’; for example, ‘{1,2,3}’ is a three-element array of
integers, ‘{{1,2}, {3,4}, {5,6}}’ is a three-by-two array,
and ‘{&"hi", &"there", &"fred"}’ is a three-element array of pointers.

15.4.1.3 C++ Expressions

GDB expression handling can interpret most C++ expressions.

Warning:GDB can only debug C++ code if you use
the proper compiler and the proper debug format. Currently,
GDB works best when debugging C++ code that is compiled
with the most recent version of GCC possible. The DWARF
debugging format is preferred; GCC defaults to this on most
popular platforms. Other compilers and/or debug formats are likely to
work badly or not at all when using GDB to debug C++
code. See Compilation.

Member function calls are allowed; you can use expressions like

count = aml->GetOriginal(x, y)

While a member function is active (in the selected stack frame), your
expressions have the same namespace available as the member function;
that is, GDB allows implicit references to the class instance
pointer this following the same rules as C++. using
declarations in the current scope are also respected by GDB.

You can call overloaded functions; GDB resolves the function
call to the right definition, with some restrictions. GDB does not
perform overload resolution involving user-defined type conversions,
calls to constructors, or instantiations of templates that do not exist
in the program. It also cannot handle ellipsis argument lists or
default arguments.

It does perform integral conversions and promotions, floating-point
promotions, arithmetic conversions, pointer conversions, conversions of
class objects to base classes, and standard conversions such as those of
functions or arrays to pointers; it requires an exact match on the
number of function arguments.

Overload resolution is always performed, unless you have specified
set overload-resolution off. See GDB Features for C++.

You must specify set overload-resolution off in order to use an
explicit function signature to call an overloaded function, as in

GDB understands variables declared as C++ lvalue or rvalue
references; you can use them in expressions just as you do in C++
source—they are automatically dereferenced.

In the parameter list shown when GDB displays a frame, the values of
reference variables are not displayed (unlike other variables); this
avoids clutter, since references are often used for large structures.
The address of a reference variable is always shown, unless
you have specified ‘set print address off’.

GDB supports the C++ name resolution operator ::—your
expressions can use it just as expressions in your program do. Since
one scope may be defined in another, you can use :: repeatedly if
necessary, for example in an expression like
‘scope1::scope2::name’. GDB also allows
resolving name scope by reference to source files, in both C and C++
debugging (see Program Variables).

15.4.1.4 C and C++ Defaults

If you allow GDB to set range checking automatically, it
defaults to off whenever the working language changes to
C or C++. This happens regardless of whether you or GDB
selects the working language.

If you allow GDB to set the language automatically, it
recognizes source files whose names end with .c, .C, or
.cc, etc, and when GDB enters code compiled from one of
these files, it sets the working language to C or C++.
See Having GDB Infer the Source Language,
for further details.

15.4.1.5 C and C++ Type and Range Checks

By default, when GDB parses C or C++ expressions, strict type
checking is used. However, if you turn type checking off, GDB
will allow certain non-standard conversions, such as promoting integer
constants to pointers.

Range checking, if turned on, is done on mathematical operations. Array
indices are not checked, since they are often used to index a pointer
that is not itself an array.

15.4.1.7 GDB Features for C++

Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++. Here is a summary:

breakpoint menus

When you want a breakpoint in a function whose name is overloaded,
GDB has the capability to display a menu of possible breakpoint
locations to help you specify which function definition you want.
See Ambiguous Expressions.

rbreak regex

Setting breakpoints using regular expressions is helpful for setting
breakpoints on overloaded functions that are not members of any special
classes.
See Setting Breakpoints.

The info vtbl command can be used to display the virtual
method tables of the object computed by expression. This shows
one entry per virtual table; there may be multiple virtual tables when
multiple inheritance is in use.

demangle name

Demangle name.
See Symbols, for a more complete description of the demangle command.

set print demangle

show print demangle

set print asm-demangle

show print asm-demangle

Control whether C++ symbols display in their source form, both when
displaying code as C++ source and when displaying disassemblies.
See Print Settings.

Control the format for printing virtual function tables.
See Print Settings.
(The vtbl commands do not work on programs compiled with the HP
ANSI C++ compiler (aCC).)

set overload-resolution on

Enable overload resolution for C++ expression evaluation. The default
is on. For overloaded functions, GDB evaluates the arguments
and searches for a function whose signature matches the argument types,
using the standard C++ conversion rules (see C++ Expressions, for details).
If it cannot find a match, it emits a message.

set overload-resolution off

Disable overload resolution for C++ expression evaluation. For
overloaded functions that are not class member functions, GDB
chooses the first function of the specified name that it finds in the
symbol table, whether or not its arguments are of the correct type. For
overloaded functions that are class member functions, GDB
searches for a function whose signature exactly matches the
argument types.

show overload-resolution

Show the current setting of overload resolution.

Overloaded symbol names

You can specify a particular definition of an overloaded symbol, using
the same notation that is used to declare such symbols in C++: type
symbol(types) rather than just symbol. You can
also use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you.
See Command Completion, for details on how to do this.

15.4.1.8 Decimal Floating Point format

GDB can examine, set and perform computations with numbers in
decimal floating point format, which in the C language correspond to the
_Decimal32, _Decimal64 and _Decimal128 types as
specified by the extension to support decimal floating-point arithmetic.

There are two encodings in use, depending on the architecture: BID (Binary
Integer Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for
PowerPC and S/390. GDB will use the appropriate encoding for the
configured target.

Because of a limitation in libdecnumber, the library used by GDB
to manipulate decimal floating point numbers, it is not possible to convert
(using a cast, for example) integers wider than 32-bit to decimal float.

In addition, in order to imitate GDB’s behaviour with binary floating
point computations, error checking in decimal float operations ignores
underflow, overflow and divide by zero exceptions.

In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect _Decimal128 values stored in floating point registers.
See PowerPC for more details.

15.4.4 Objective-C

This section provides information about some commands and command
options that are useful for debugging Objective-C code. See also
info classes, and info selectors, for a
few more commands specific to Objective-C support.

15.4.4.1 Method Names in Commands

The following commands have been extended to accept Objective-C method
names as line specifications:

clear

break

info line

jump

list

A fully qualified Objective-C method name is specified as

-[ClassmethodName]

where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name Class and method name methodName are enclosed in
brackets, similar to the way messages are specified in Objective-C
source code. For example, to set a breakpoint at the create
instance method of class Fruit in the program currently being
debugged, enter:

break -[Fruit create]

To list ten program lines around the initialize class method,
enter:

list +[NSText initialize]

In the current version of GDB, the plus or minus sign is
required. In future versions of GDB, the plus or minus
sign will be optional, but you can use it to narrow the search. It
is also possible to specify just a method name:

break create

You must specify the complete method name, including any colons. If
your program’s source files contain more than one create method,
you’ll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type ‘0’ to exit if
none apply.

As another example, to clear a breakpoint established at the
makeKeyAndOrderFront: method of the NSWindow class, enter:

15.4.4.2 The Print Command With Objective-C

The print command has also been extended to accept methods. For example:

print -[object hash]

will tell GDB to send the hash message to object
and print the result. Also, an additional command has been added,
print-object or po for short, which is meant to print
the description of an object. However, this command may only work
with certain Objective-C libraries that have a particular hook
function, _NSPrintForDebugger, defined.

15.4.5 OpenCL C

15.4.5.1 OpenCL C Datatypes

GDB supports the builtin scalar and vector datatypes specified
by OpenCL 1.1. In addition the half- and double-precision floating point
data types of the cl_khr_fp16 and cl_khr_fp64 OpenCL
extensions are also known to GDB.

15.4.5.3 OpenCL C Operators

15.4.6 Fortran

GDB can be used to debug programs written in Fortran, but it
currently supports only the features of Fortran 77 language.

Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers
among them) append an underscore to the names of variables and
functions. When you debug programs compiled by those compilers, you
will need to refer to variables and functions with a trailing
underscore.

15.4.6.1 Fortran Operators and Expressions

Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on characters or other non-
arithmetic types. Operators are often defined on groups of types.

**

The exponentiation operator. It raises the first operand to the power
of the second one.

:

The range operator. Normally used in the form of array(low:high) to
represent a section of array.

%

The access component operator. Normally used to access elements in derived
types. Also suitable for unions. As unions aren’t part of regular Fortran,
this can only happen when accessing a register that uses a gdbarch-defined
union type.

15.4.6.2 Fortran Defaults

Fortran symbols are usually case-insensitive, so GDB by
default uses case-insensitive matches for Fortran symbols. You can
change that with the ‘set case-insensitive’ command, see
Symbols, for the details.

15.4.7 Pascal

Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work. GDB does not support
entering expressions, printing values, or similar features using Pascal
syntax.

The Pascal-specific command set print pascal_static-members
controls whether static members of Pascal objects are displayed.
See pascal_static-members.

15.4.8 Rust

GDB supports the Rust
Programming Language. Type- and value-printing, and expression
parsing, are reasonably complete. However, there are a few
peculiarities and holes to be aware of.

Linespecs (see Specify Location) are never relative to the current
crate. Instead, they act as if there were a global namespace of
crates, somewhat similar to the way extern crate behaves.

That is, if GDB is stopped at a breakpoint in a function in
crate ‘A’, module ‘B’, then break B::f will attempt
to set a breakpoint in a function named ‘f’ in a crate named
‘B’.

As a consequence of this approach, linespecs also cannot refer to
items using ‘self::’ or ‘super::’.

Because GDB implements Rust name-lookup semantics in
expressions, it will sometimes prepend the current crate to a name.
For example, if GDB is stopped at a breakpoint in the crate
‘K’, then print ::x::y will try to find the symbol
‘K::x::y’.

However, since it is useful to be able to refer to other crates when
debugging, GDB provides the extern extension to
circumvent this. To use the extension, just put extern before
a path expression to refer to the otherwise unavailable “global”
scope.

In the above example, if you wanted to refer to the symbol ‘y’ in
the crate ‘x’, you would use print extern x::y.

The Rust expression evaluator does not support “statement-like”
expressions such as if or match, or lambda expressions.

Tuple expressions are not implemented.

The Rust expression evaluator does not currently implement the
Drop trait. Objects that may be created by the evaluator will
never be destroyed.

GDB does not implement type inference for generics. In order
to call generic functions or otherwise refer to generic items, you
will have to specify the type parameters manually.

GDB currently uses the C++ demangler for Rust. In most
cases this does not cause any problems. However, in an expression
context, completing a generic function name will give syntactically
invalid results. This happens because Rust requires the ‘::’
operator between the function name and its generic arguments. For
example, GDB might provide a completion like
crate::f<u32>, where the parser would require
crate::f::<u32>.

As of this writing, the Rust compiler (version 1.8) has a few holes in
the debugging information it generates. These holes prevent certain
features from being implemented by GDB:

Method calls cannot be made via traits.

Operator overloading is not implemented.

When debugging in a monomorphized function, you cannot use the generic
type names.

The type Self is not available.

use statements are not available, so some names may not be
available in the crate.

15.4.9 Modula-2

The extensions made to GDB to support Modula-2 only support
output from the GNU Modula-2 compiler (which is currently being
developed). Other Modula-2 compilers are not currently supported, and
attempting to debug executables produced by them is most likely
to give an error as GDB reads in the executable’s symbol
table.

15.4.9.1 Operators

Operators must be defined on values of specific types. For instance,
+ is defined on numbers, but not on structures. Operators are
often defined on groups of types. For the purposes of Modula-2, the
following definitions hold:

Integral types consist of INTEGER, CARDINAL, and
their subranges.

Character types consist of CHAR and its subranges.

Floating-point types consist of REAL.

Pointer types consist of anything declared as POINTER TO
type.

Scalar types consist of all of the above.

Set types consist of SET and BITSET types.

Boolean types consist of BOOLEAN.

The following operators are supported, and appear in order of
increasing precedence:

,

Function argument or array index separator.

:=

Assignment. The value of var:=value is
value.

<, >

Less than, greater than on integral, floating-point, or enumerated
types.

<=, >=

Less than or equal to, greater than or equal to
on integral, floating-point and enumerated types, or set inclusion on
set types. Same precedence as <.

=, <>, #

Equality and two ways of expressing inequality, valid on scalar types.
Same precedence as <. In GDB scripts, only <> is
available for inequality, since # conflicts with the script
comment character.

IN

Set membership. Defined on set types and the types of their members.
Same precedence as <.

15.4.9.2 Built-in Functions and Procedures

Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:

a

represents an ARRAY variable.

c

represents a CHAR constant or variable.

i

represents a variable or constant of integral type.

m

represents an identifier that belongs to a set. Generally used in the
same function with the metavariable s. The type of s should
be SET OF mtype (where mtype is the type of m).

n

represents a variable or constant of integral or floating-point type.

r

represents a variable or constant of floating-point type.

t

represents a type.

v

represents a variable.

x

represents a variable or constant of one of many types. See the
explanation of the function for details.

All Modula-2 built-in procedures also return a result, described below.

ABS(n)

Returns the absolute value of n.

CAP(c)

If c is a lower case letter, it returns its upper case
equivalent, otherwise it returns its argument.

CHR(i)

Returns the character whose ordinal value is i.

DEC(v)

Decrements the value in the variable v by one. Returns the new value.

DEC(v,i)

Decrements the value in the variable v by i. Returns the
new value.

EXCL(m,s)

Removes the element m from the set s. Returns the new
set.

FLOAT(i)

Returns the floating point equivalent of the integer i.

HIGH(a)

Returns the index of the last member of a.

INC(v)

Increments the value in the variable v by one. Returns the new value.

INC(v,i)

Increments the value in the variable v by i. Returns the
new value.

INCL(m,s)

Adds the element m to the set s if it is not already
there. Returns the new set.

MAX(t)

Returns the maximum value of the type t.

MIN(t)

Returns the minimum value of the type t.

ODD(i)

Returns boolean TRUE if i is an odd number.

ORD(x)

Returns the ordinal value of its argument. For example, the ordinal
value of a character is its ASCII value (on machines supporting
the ASCII character set). The argument x must be of an
ordered type, which include integral, character and enumerated types.

SIZE(x)

Returns the size of its argument. The argument x can be a
variable or a type.

TRUNC(r)

Returns the integral part of r.

TSIZE(x)

Returns the size of its argument. The argument x can be a
variable or a type.

VAL(t,i)

Returns the member of the type t whose ordinal value is i.

Warning: Sets and their operations are not yet supported, so
GDB treats the use of procedures INCL and EXCL as
an error.

15.4.9.3 Constants

GDB allows you to express the constants of Modula-2 in the following
ways:

Integer constants are simply a sequence of digits. When used in an
expression, a constant is interpreted to be type-compatible with the
rest of the expression. Hexadecimal integers are specified by a
trailing ‘H’, and octal integers by a trailing ‘B’.

Floating point constants appear as a sequence of digits, followed by a
decimal point and another sequence of digits. An optional exponent can
then be specified, in the form ‘E[+|-]nnn’, where
‘[+|-]nnn’ is the desired exponent. All of the
digits of the floating point constant must be valid decimal (base 10)
digits.

Character constants consist of a single character enclosed by a pair of
like quotes, either single (') or double ("). They may
also be expressed by their ordinal value (their ASCII value, usually)
followed by a ‘C’.

String constants consist of a sequence of characters enclosed by a
pair of like quotes, either single (') or double (").
Escape sequences in the style of C are also allowed. See C and C++ Constants, for a brief explanation of escape
sequences.

15.4.9.4 Modula-2 Types

Currently GDB can print the following data types in Modula-2
syntax: array types, record types, set types, pointer types, procedure
types, enumerated types, subrange types and base types. You can also
print the contents of variables declared using these type.
This section gives a number of simple source code examples together with
sample GDB sessions.

Note that at present you cannot interactively manipulate set
expressions using the debugger.

The following example shows how you might declare an array in Modula-2
and how you can interact with GDB to print its type and contents:

VAR
s: ARRAY [-10..10] OF CHAR ;

(gdb) ptype s
ARRAY [-10..10] OF CHAR

Note that the array handling is not yet complete and although the type
is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not -10 as in the example
above.

15.4.9.5 Modula-2 Defaults

If type and range checking are set automatically by GDB, they
both default to on whenever the working language changes to
Modula-2. This happens regardless of whether you or GDB
selected the working language.

If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with .mod sets the
working language to Modula-2. See Having GDB
Infer the Source Language, for further details.

15.4.9.6 Deviations from Standard Modula-2

A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:

Unlike in standard Modula-2, pointer constants can be formed by
integers. This allows you to modify pointer variables during
debugging. (In standard Modula-2, the actual address contained in a
pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or expression that
returned a pointer.)

C escape sequences can be used in strings and characters to represent
non-printable characters. GDB prints out strings with these
escape sequences embedded. Single non-printable characters are
printed using the ‘CHR(nnn)’ format.

The assignment operator (:=) returns the value of its right-hand
argument.

15.4.9.8 The Scope Operators :: and .

There are a few subtle differences between the Modula-2 scope operator
(.) and the GDB scope operator (::). The two have
similar syntax:

module . idscope :: id

where scope is the name of a module or a procedure,
module the name of a module, and id is any declared
identifier within your program, except another module.

Using the :: operator makes GDB search the scope
specified by scope for the identifier id. If it is not
found in the specified scope, then GDB searches all scopes
enclosing the one specified by scope.

Using the . operator makes GDB search the current scope for
the identifier specified by id that was imported from the
definition module specified by module. With this operator, it is
an error if the identifier id was not imported from definition
module module, or if id is not an identifier in
module.

15.4.9.9 GDB and Modula-2

Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of set print and show print apply
specifically to C and C++: ‘vtbl’, ‘demangle’,
‘asm-demangle’, ‘object’, and ‘union’. The first four
apply to C++, and the last to the C union type, which has no direct
analogue in Modula-2.

The @ operator (see Expressions), while available
with any language, is not useful with Modula-2. Its
intent is to aid the debugging of dynamic arrays, which cannot be
created in Modula-2 as they can in C or C++. However, because an
address can be specified by an integral constant, the construct
‘{type}adrexp’ is still useful.

In GDB scripts, the Modula-2 inequality operator # is
interpreted as the beginning of a comment. Use <> instead.

15.4.10 Ada

The extensions made to GDB for Ada only support
output from the GNU Ada (GNAT) compiler.
Other Ada compilers are not currently supported, and
attempting to debug executables produced by them is most likely
to be difficult.

15.4.10.1 Introduction

The Ada mode of GDB supports a fairly large subset of Ada expression
syntax, with some extensions.
The philosophy behind the design of this subset is

That GDB should provide basic literals and access to operations for
arithmetic, dereferencing, field selection, indexing, and subprogram calls,
leaving more sophisticated computations to subprograms written into the
program (which therefore may be called from GDB).

That type safety and strict adherence to Ada language restrictions
are not particularly important to the GDB user.

That brevity is important to the GDB user.

Thus, for brevity, the debugger acts as if all names declared in
user-written packages are directly visible, even if they are not visible
according to Ada rules, thus making it unnecessary to fully qualify most
names with their packages, regardless of context. Where this causes
ambiguity, GDB asks the user’s intent.

The debugger will start in Ada mode if it detects an Ada main program.
As for other languages, it will enter Ada mode when stopped in a program that
was translated from an Ada source file.

While in Ada mode, you may use ‘--’ for comments. This is useful
mostly for documenting command files. The standard GDB comment
(‘#’) still works at the beginning of a line in Ada mode, but not in the
middle (to allow based literals).

The names in
Characters.Latin_1 are not available and
concatenation is not implemented. Thus, escape characters in strings are
not currently available.

Equality tests (‘=’ and ‘/=’) on arrays test for bitwise
equality of representations. They will generally work correctly
for strings and arrays whose elements have integer or enumeration types.
They may not work correctly for arrays whose element
types have user-defined equality, for arrays of real values
(in particular, IEEE-conformant floating point, because of negative
zeroes and NaNs), and for arrays whose elements contain unused bits with
indeterminate values.

The other component-by-component array operations (and, or,
xor, not, and relational tests other than equality)
are not implemented.

There is limited support for array and record aggregates. They are
permitted only on the right sides of assignments, as in these examples:

Changing a
discriminant’s value by assigning an aggregate has an
undefined effect if that discriminant is used within the record.
However, you can first modify discriminants by directly assigning to
them (which normally would not be allowed in Ada), and then performing an
aggregate assignment. For example, given a variable A_Rec
declared to have a type such as:

As this example also illustrates, GDB is very loose about the usual
rules concerning aggregates. You may leave out some of the
components of an array or record aggregate (such as the Len
component in the assignment to A_Rec above); they will retain their
original values upon assignment. You may freely use dynamic values as
indices in component associations. You may even use overlapping or
redundant component associations, although which component values are
assigned in such cases is not defined.

Calls to dispatching subprograms are not implemented.

The overloading algorithm is much more limited (i.e., less selective)
than that of real Ada. It makes only limited use of the context in
which a subexpression appears to resolve its meaning, and it is much
looser in its rules for allowing type matches. As a result, some
function calls will be ambiguous, and the user will be asked to choose
the proper resolution.

The new operator is not implemented.

Entry calls are not implemented.

Aside from printing, arithmetic operations on the native VAX floating-point
formats are not supported.

It is not possible to slice a packed array.

The names True and False, when not part of a qualified name,
are interpreted as if implicitly prefixed by Standard, regardless of
context.
Should your program
redefine these names in a package or procedure (at best a dubious practice),
you will have to use fully qualified names to access their new definitions.

15.4.10.3 Additions to Ada

As it does for other languages, GDB makes certain generic
extensions to Ada (see Expressions):

If the expression E is a variable residing in memory (typically
a local variable or array element) and N is a positive integer,
then E@N displays the values of E and the
N-1 adjacent variables following it in memory as an array. In
Ada, this operator is generally not necessary, since its prime use is
in displaying parts of an array, and slicing will usually do this in
Ada. However, there are occasional uses when debugging programs in
which certain debugging information has been optimized away.

B::var means “the variable named var that
appears in function or file B.” When B is a file name,
you must typically surround it in single quotes.

In addition, GDB provides a few other shortcuts and outright
additions specific to Ada:

The assignment statement is allowed as an expression, returning
its right-hand operand as its value. Thus, you may enter

(gdb) set x := y + 3
(gdb) print A(tmp := y + 1)

The semicolon is allowed as an “operator,” returning as its value
the value of its right-hand operand.
This allows, for example,
complex conditional breaks:

(gdb) break f
(gdb) condition 1 (report(i); k += 1; A(k) > 100)

Rather than use catenation and symbolic character names to introduce special
characters into strings, one may instead use a special bracket notation,
which is also used to print strings. A sequence of characters of the form
‘["XX"]’ within a string or character literal denotes the
(single) character whose numeric encoding is XX in hexadecimal. The
sequence of characters ‘["""]’ also denotes a single quotation mark
in strings. For example,

"One line.["0a"]Next line.["0a"]"

contains an ASCII newline character (Ada.Characters.Latin_1.LF)
after each period.

The subtype used as a prefix for the attributes 'Pos, 'Min, and
'Max is optional (and is ignored in any case). For example, it is valid
to write

(gdb) print 'max(x, y)

When printing arrays, GDB uses positional notation when the
array has a lower bound of 1, and uses a modified named notation otherwise.
For example, a one-dimensional array of three integers with a lower bound
of 3 might print as

(3 => 10, 17, 1)

That is, in contrast to valid Ada, only the first component has a =>
clause.

You may abbreviate attributes in expressions with any unique,
multi-character subsequence of
their names (an exact match gets preference).
For example, you may use a'len, a'gth, or a'lh
in place of a'length.

Since Ada is case-insensitive, the debugger normally maps identifiers you type
to lower case. The GNAT compiler uses upper-case characters for
some of its internal identifiers, which are normally of no interest to users.
For the rare occasions when you actually have to look at them,
enclose them in angle brackets to avoid the lower-case mapping.
For example,

(gdb) print <JMPBUF_SAVE>[0]

Printing an object of class-wide type or dereferencing an
access-to-class-wide value will display all the components of the object’s
specific type (as indicated by its run-time tag). Likewise, component
selection on such a value will operate on the specific type of the
object.

15.4.10.4 Overloading support for Ada

The debugger supports limited overloading. Given a subprogram call in which
the function symbol has multiple definitions, it will use the number of
actual parameters and some information about their types to attempt to narrow
the set of definitions. It also makes very limited use of context, preferring
procedures to functions in the context of the call command, and
functions to procedures elsewhere.

If, after narrowing, the set of matching definitions still contains more than
one definition, GDB will display a menu to query which one it should
use, for instance:

15.4.10.5 Stopping at the Very Beginning

It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure.
As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
adainit. To run your program up to the beginning of
elaboration, simply use the following two commands:
tbreak adainit and run.

15.4.10.6 Ada Exceptions

A command is provided to list all Ada exceptions:

info exceptions

info exceptions regexp

The info exceptions command allows you to list all Ada exceptions
defined within the program being debugged, as well as their addresses.
With a regular expression, regexp, as argument, only those exceptions
whose names match regexp are listed.

Below is a small example, showing how the command can be used, first
without argument, and next with a regular expression passed as an
argument.

These commands are like the break … thread …
command (see Thread Stops). The
location argument specifies source lines, as described
in Specify Location.

Use the qualifier ‘task taskno’ with a breakpoint command
to specify that you only want GDB to stop the program when a
particular Ada task reaches this breakpoint. The taskno is one of the
numeric task identifiers assigned by GDB, shown in the first
column of the ‘info tasks’ display.

If you do not specify ‘task taskno’ when you set a
breakpoint, the breakpoint applies to all tasks of your
program.

You can use the task qualifier on conditional breakpoints as
well; in this case, place ‘task taskno’ before the
breakpoint condition (before the if).

15.4.10.8 Tasking Support when Debugging Core Files

When inspecting a core file, as opposed to debugging a live program,
tasking support may be limited or even unavailable, depending on
the platform being used.
For instance, on x86-linux, the list of tasks is available, but task
switching is not supported.

On certain platforms, the debugger needs to perform some
memory writes in order to provide Ada tasking support. When inspecting
a core file, this means that the core file must be opened with read-write
privileges, using the command ‘"set write on"’ (see Patching).
Under these circumstances, you should make a backup copy of the core
file before inspecting it with GDB.

15.4.10.9 Tasking Support when using the Ravenscar Profile

The Ravenscar Profile is a subset of the Ada tasking features,
specifically designed for systems with safety-critical real-time
requirements.

set ravenscar task-switching on

Allows task switching when debugging a program that uses the Ravenscar
Profile. This is the default.

set ravenscar task-switching off

Turn off task switching when debugging a program that uses the Ravenscar
Profile. This is mostly intended to disable the code that adds support
for the Ravenscar Profile, in case a bug in either GDB or in
the Ravenscar runtime is preventing GDB from working properly.
To be effective, this command should be run before the program is started.

show ravenscar task-switching

Show whether it is possible to switch from task to task in a program
using the Ravenscar Profile.

15.4.10.10 Ada Settings

set varsize-limit size

Prevent GDB from attempting to evaluate objects whose size
is above the given limit (size) when those sizes are computed
from run-time quantities. This is typically the case when the object
has a variable size, such as an array whose bounds are not known at
compile time for example. Setting size to unlimited
removes the size limitation. By default, the limit is about 65KB.

The purpose of having such a limit is to prevent GDB from
trying to grab enormous chunks of virtual memory when asked to evaluate
a quantity whose bounds have been corrupted or have not yet been fully
initialized. The limit applies to the results of some subexpressions
as well as to complete expressions. For example, an expression denoting
a simple integer component, such as x.y.z, may fail if the size of
x.y is variable and exceeds size. On the other hand,
GDB is sometimes clever; the expression A(i), where
A is an array variable with non-constant size, will generally
succeed regardless of the bounds on A, as long as the component
size is less than size.

show varsize-limit

Show the limit on types whose size is determined by run-time quantities.

15.4.10.11 Known Peculiarities of Ada Mode

Besides the omissions listed previously (see Omissions from Ada),
we know of several problems with and limitations of Ada mode in
GDB,
some of which will be fixed with planned future releases of the debugger
and the GNU Ada compiler.

Static constants that the compiler chooses not to materialize as objects in
storage are invisible to the debugger.

Named parameter associations in function argument lists are ignored (the
argument lists are treated as positional).

Many useful library packages are currently invisible to the debugger.

Fixed-point arithmetic, conversions, input, and output is carried out using
floating-point arithmetic, and may give results that only approximate those on
the host machine.

The GNAT compiler never generates the prefix Standard for any of
the standard symbols defined by the Ada language. GDB knows about
this: it will strip the prefix from names when you use it, and will never
look for a name you have so qualified among local symbols, nor match against
symbols in other packages or subprograms. If you have
defined entities anywhere in your program other than parameters and
local variables whose simple names match names in Standard,
GNAT’s lack of qualification here can cause confusion. When this happens,
you can usually resolve the confusion
by qualifying the problematic names with package
Standard explicitly.

Older versions of the compiler sometimes generate erroneous debugging
information, resulting in the debugger incorrectly printing the value
of affected entities. In some cases, the debugger is able to work
around an issue automatically. In other cases, the debugger is able
to work around the issue, but the work-around has to be specifically
enabled.

set ada trust-PAD-over-XVS on

Configure GDB to strictly follow the GNAT encoding when computing the
value of Ada entities, particularly when PAD and PAD___XVS
types are involved (see ada/exp_dbug.ads in the GCC sources for
a complete description of the encoding used by the GNAT compiler).
This is the default.

set ada trust-PAD-over-XVS off

This is related to the encoding using by the GNAT compiler. If GDB
sometimes prints the wrong value for certain entities, changing ada
trust-PAD-over-XVS to off activates a work-around which may fix
the issue. It is always safe to set ada trust-PAD-over-XVS to
off, but this incurs a slight performance penalty, so it is
recommended to leave this setting to on unless necessary.

Internally, the debugger also relies on the compiler following a number
of conventions known as the ‘GNAT Encoding’, all documented in
gcc/ada/exp_dbug.ads in the GCC sources. This encoding describes
how the debugging information should be generated for certain types.
In particular, this convention makes use of descriptive types,
which are artificial types generated purely to help the debugger.

These encodings were defined at a time when the debugging information
format used was not powerful enough to describe some of the more complex
types available in Ada. Since DWARF allows us to express nearly all
Ada features, the long-term goal is to slowly replace these descriptive
types by their pure DWARF equivalent. To facilitate that transition,
a new maintenance option is available to force the debugger to ignore
those descriptive types. It allows the user to quickly evaluate how
well GDB works without them.

maintenance ada set ignore-descriptive-types [on|off]

Control whether the debugger should ignore descriptive types.
The default is not to ignore descriptives types (off).

15.5 Unsupported Languages

In addition to the other fully-supported programming languages,
GDB also provides a pseudo-language, called minimal.
It does not represent a real programming language, but provides a set
of capabilities close to what the C or assembly languages provide.
This should allow most simple operations to be performed while debugging
an application that uses a language currently not supported by GDB.

If the language is set to auto, GDB will automatically
select this language if the current frame corresponds to an unsupported
language.

16 Examining the Symbol Table

The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program. This information is inherent in the text of your program and
does not change as your program executes. GDB finds it in your
program’s symbol table, in the file indicated when you started GDB
(see Choosing Files), or by one of the
file-management commands (see Commands to Specify Files).

Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters. The
most frequent case is in referring to static variables in other
source files (see Program Variables). File names
are recorded in object files as debugging symbols, but GDB would
ordinarily parse a typical file name, like foo.c, as the three words
‘foo’ ‘.’ ‘c’. To allow GDB to recognize
‘foo.c’ as a single symbol, enclose it in single quotes; for example,

p 'foo.c'::x

looks up the value of x in the scope of the file foo.c.

set case-sensitive on

set case-sensitive off

set case-sensitive auto

Normally, when GDB looks up symbols, it matches their names
with case sensitivity determined by the current source language.
Occasionally, you may wish to control that. The command set
case-sensitive lets you do that by specifying on for
case-sensitive matches or off for case-insensitive ones. If
you specify auto, case sensitivity is reset to the default
suitable for the source language. The default is case-sensitive
matches for all languages except for Fortran, for which the default is
case-insensitive matches.

show case-sensitive

This command shows the current setting of case sensitivity for symbols
lookups.

set print type methods

set print type methods on

set print type methods off

Normally, when GDB prints a class, it displays any methods
declared in that class. You can control this behavior either by
passing the appropriate flag to ptype, or using set
print type methods. Specifying on will cause GDB to
display the methods; this is the default. Specifying off will
cause GDB to omit the methods.

show print type methods

This command shows the current setting of method display when printing
classes.

set print type nested-type-limit limit

set print type nested-type-limit unlimited

Set the limit of displayed nested types that the type printer will
show. A limit of unlimited or -1 will show all
nested definitions. By default, the type printer will not show any nested
types defined in classes.

show print type nested-type-limit

This command shows the current display limit of nested types when
printing classes.

set print type typedefs

set print type typedefs on

set print type typedefs off

Normally, when GDB prints a class, it displays any typedefs
defined in that class. You can control this behavior either by
passing the appropriate flag to ptype, or using set
print type typedefs. Specifying on will cause GDB to
display the typedef definitions; this is the default. Specifying
off will cause GDB to omit the typedef definitions.
Note that this controls whether the typedef definition itself is
printed, not whether typedef names are substituted when printing other
types.

show print type typedefs

This command shows the current setting of typedef display when
printing classes.

info address symbol

Describe where the data for symbol is stored. For a register
variable, this says which register it is kept in. For a non-register
local variable, this prints the stack-frame offset at which the variable
is always stored.

Note the contrast with ‘print &symbol’, which does not work
at all for a register variable, and for a stack local variable prints
the exact address of the current instantiation of the variable.

info symbol addr

Print the name of a symbol which is stored at the address addr.
If no symbol is stored exactly at addr, GDB prints the
nearest symbol and an offset from it:

(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text

This is the opposite of the info address command. You can use
it to find out the name of a variable or a function given its address.

For dynamically linked executables, the name of executable or shared
library containing the symbol is also printed:

Demangle name.
If language is provided it is the name of the language to demangle
name in. Otherwise name is demangled in the current language.

The ‘--’ option specifies the end of options,
and is useful when name begins with a dash.

The parameter demangle-style specifies how to interpret the kind
of mangling used. See Print Settings.

whatis[/flags] [arg]

Print the data type of arg, which can be either an expression
or a name of a data type. With no argument, print the data type of
$, the last value in the value history.

If arg is an expression (see Expressions), it
is not actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place.

If arg is a variable or an expression, whatis prints its
literal type as it is used in the source code. If the type was
defined using a typedef, whatis will not print
the data type underlying the typedef. If the type of the
variable or the expression is a compound data type, such as
struct or class, whatis never prints their
fields or methods. It just prints the struct/class
name (a.k.a. its tag). If you want to see the members of
such a compound data type, use ptype.

If arg is a type name that was defined using typedef,
whatisunrolls only one level of that typedef.
Unrolling means that whatis will show the underlying type used
in the typedef declaration of arg. However, if that
underlying type is also a typedef, whatis will not
unroll it.

For C code, the type names may also have the form ‘class
class-name’, ‘struct struct-tag’, ‘union
union-tag’ or ‘enum enum-tag’.

flags can be used to modify how the type is displayed.
Available flags are:

Notice the format of the first column of comments. There, you can
find two parts separated by the ‘|’ character: the offset,
which indicates where the field is located inside the struct, in
bytes, and the size of the field. Another interesting line is
the marker of a hole in the struct, indicating that it may be
possible to pack the struct and make it use less space by reorganizing
its fields.

In this case, since struct tuv and struct xyz occupy the
same space (because we are dealing with an union), the offset is not
printed for them. However, you can still examine the offset of each
of these structures’ fields.

Another useful scenario is printing the offsets of a struct containing
bitfields:

Note how the offset information is now extended to also include how
many bits are left to be used in each bitfield.

ptype[/flags] [arg]

ptype accepts the same arguments as whatis, but prints a
detailed description of the type, instead of just the name of the type.
See Expressions.

Contrary to whatis, ptype always unrolls any
typedefs in its argument declaration, whether the argument is
a variable, expression, or a data type. This means that ptype
of a variable or an expression will not print literally its type as
present in the source code—use whatis for that. typedefs at
the pointer or reference targets are also unrolled. Only typedefs of
fields, methods and inner class typedefs of structs,
classes and unions are not unrolled even with ptype.

As with whatis, using ptype without an argument refers to
the type of $, the last value in the value history.

Sometimes, programs use opaque data types or incomplete specifications
of complex data structure. If the debug information included in the
program does not allow GDB to display a full declaration of
the data type, it will say ‘<incomplete type>’. For example,
given these declarations:

struct foo;
struct foo *fooptr;

but no definition for struct foo itself, GDB will say:

(gdb) ptype foo
$1 = <incomplete type>

“Incomplete type” is C terminology for data types that are not
completely specified.

Othertimes, information about a variable’s type is completely absent
from the debug information included in the program. This most often
happens when the program or library where the variable is defined
includes no debug information at all. GDB knows the variable
exists from inspecting the linker/loader symbol table (e.g., the ELF
dynamic symbol table), but such symbols do not contain type
information. Inspecting the type of a (global) variable for which
GDB has no type information shows:

Print a brief description of all types whose names match the regular
expression regexp (or all types in your program, if you supply
no argument). Each complete typename is matched as though it were a
complete line; thus, ‘i type value’ gives information on all
types in your program whose names include the string value, but
‘i type ^value$’ gives information only on types whose complete
name is value.

This command differs from ptype in two ways: first, like
whatis, it does not print a detailed description; second, it
lists all source files and line numbers where a type is defined.

info type-printers

Versions of GDB that ship with Python scripting enabled may
have “type printers” available. When using ptype or
whatis, these printers are consulted when the name of a type
is needed. See Type Printing API, for more information on writing
type printers.

info type-printers displays all the available type printers.

enable type-printer name…

disable type-printer name…

These commands can be used to enable or disable type printers.

info scope location

List all the variables local to a particular scope. This command
accepts a location argument—a function name, a source line, or
an address preceded by a ‘*’, and prints all the variables local
to the scope defined by that location. (See Specify Location, for
details about supported forms of location.) For example:

(gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.

This command is especially useful for determining what data to collect
during a trace experiment, see collect.

info source

Show information about the current source file—that is, the source file for
the function containing the current point of execution:

the name of the source file, and the directory containing it,

the directory it was compiled in,

its length, in lines,

which programming language it is written in,

if the debug information provides it, the program that compiled the file
(which may include, e.g., the compiler version and command line arguments),

whether the executable includes debugging information for that file, and
if so, what format the information is in (e.g., STABS, Dwarf 2, etc.), and

whether the debugging information includes information about
preprocessor macros.

info sources

Print the names of all source files in your program for which there is
debugging information, organized into two lists: files whose symbols
have already been read, and files whose symbols will be read when needed.

info functions

Print the names and data types of all defined functions.
Similarly to ‘info types’, this command groups its output by source
files and annotates each function definition with its source line
number.

info functions regexp

Like ‘info functions’, but only print the names and data types of
functions whose names contain a match for regular expression
regexp. Thus, ‘info fun step’ finds all functions whose
names include step; ‘info fun ^step’ finds those whose names
start with step. If a function name contains characters that
conflict with the regular expression language (e.g.
‘operator*()’), they may be quoted with a backslash.

info variables

Print the names and data types of all variables that are defined
outside of functions (i.e. excluding local variables).
The printed variables are grouped by source files and annotated with
their respective source line numbers.

info variables regexp

Like info variables, but only print the names and data types of
non-local variables whose names contain a match for regular expression
regexp.

info classes

info classes regexp

Display all Objective-C classes in your program, or
(with the regexp argument) all those matching a particular regular
expression.

info selectors

info selectors regexp

Display all Objective-C selectors in your program, or
(with the regexp argument) all those matching a particular regular
expression.

set opaque-type-resolution on

Tell GDB to resolve opaque types. An opaque type is a type
declared as a pointer to a struct, class, or
union—for example, struct MyType *—that is used in one
source file although the full declaration of struct MyType is in
another source file. The default is on.

A change in the setting of this subcommand will not take effect until
the next time symbols for a file are loaded.

set opaque-type-resolution off

Tell GDB not to resolve opaque types. In this case, the type
is printed as follows:

{<no data fields>}

show opaque-type-resolution

Show whether opaque types are resolved or not.

set print symbol-loading

set print symbol-loading full

set print symbol-loading brief

set print symbol-loading off

The set print symbol-loading command allows you to control the
printing of messages when GDB loads symbol information.
By default a message is printed for the executable and one for each
shared library, and normally this is what you want. However, when
debugging apps with large numbers of shared libraries these messages
can be annoying.
When set to brief a message is printed for each executable,
and when GDB loads a collection of shared libraries at once
it will only print one message regardless of the number of shared
libraries. When set to off no messages are printed.

show print symbol-loading

Show whether messages will be printed when a GDB command
entered from the keyboard causes symbol information to be loaded.

maint print symbols [-pc address][filename]

maint print symbols [-objfile objfile][-source source][--][filename]

maint print psymbols [-objfile objfile][-pc address][--][filename]

maint print psymbols [-objfile objfile][-source source][--][filename]

maint print msymbols [-objfile objfile][--][filename]

Write a dump of debugging symbol data into the file filename or
the terminal if filename is unspecified.
If -objfile objfile is specified, only dump symbols for
that objfile.
If -pc address is specified, only dump symbols for the file
with code at that address. Note that address may be a symbol like
main.
If -source source is specified, only dump symbols for that
source file.

These commands are used to debug the GDB symbol-reading code.
These commands do not modify internal GDB state, therefore
‘maint print symbols’ will only print symbols for already expanded symbol
tables.
You can use the command info sources to find out which files these are.
If you use ‘maint print psymbols’ instead, the dump shows information
about symbols that GDB only knows partially—that is, symbols
defined in files that GDB has skimmed, but not yet read completely.
Finally, ‘maint print msymbols’ just dumps “minimal symbols”, e.g.,
“ELF symbols”.

List the struct symtab or struct partial_symtab
structures whose names match regexp. If regexp is not
given, list them all. The output includes expressions which you can
copy into a GDB debugging this one to examine a particular
structure in more detail. For example:

We see that there is one partial symbol table whose filename contains
the string ‘dwarf2read’, belonging to the ‘gdb’ executable;
and we see that GDB has not read in any symtabs yet at all.
If we set a breakpoint on a function, that will cause GDB to
read the symtab for the compilation unit containing that function:

17 Altering Execution

Once you think you have found an error in your program, you might want to
find out for certain whether correcting the apparent error would lead to
correct results in the rest of the run. You can find the answer by
experiment, using the GDB features for altering execution of the
program.

For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.

17.1 Assignment to Variables

To alter the value of a variable, evaluate an assignment expression.
See Expressions. For example,

print x=4

stores the value 4 into the variable x, and then prints the
value of the assignment expression (which is 4).
See Using GDB with Different Languages, for more
information on operators in supported languages.

If you are not interested in seeing the value of the assignment, use the
set command instead of the print command. set is
really the same as print except that the expression’s value is
not printed and is not put in the value history (see Value History). The expression is evaluated only for its effects.

If the beginning of the argument string of the set command
appears identical to a set subcommand, use the set
variable command instead of just set. This command is identical
to set except for its lack of subcommands. For example, if your
program has a variable width, you get an error if you try to set
a new value with just ‘set width=13’, because GDB has the
command set width:

The invalid expression, of course, is ‘=47’. In
order to actually set the program’s variable width, use

(gdb) set var width=47

Because the set command has many subcommands that can conflict
with the names of program variables, it is a good idea to use the
set variable command instead of just set. For example, if
your program has a variable g, you run into problems if you try
to set a new value with just ‘set g=4’, because GDB has
the command set gnutarget, abbreviated set g:

The program variable g did not change, and you silently set the
gnutarget to an invalid value. In order to set the variable
g, use

(gdb) set var g=4

GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa,
and you can convert any structure to any other structure that is the
same length or shorter.

To store values into arbitrary places in memory, use the ‘{…}’
construct to generate a value of specified type at a specified address
(see Expressions). For example, {int}0x83040 refers
to memory location 0x83040 as an integer (which implies a certain size
and representation in memory), and

17.2 Continuing at a Different Address

Ordinarily, when you continue your program, you do so at the place where
it stopped, with the continue command. You can instead continue at
an address of your own choosing, with the following commands:

jump location

j location

Resume execution at location. Execution stops again immediately
if there is a breakpoint there. See Specify Location, for a description
of the different forms of location. It is common
practice to use the tbreak command in conjunction with
jump. See Setting Breakpoints.

The jump command does not change the current stack frame, or
the stack pointer, or the contents of any memory location or any
register other than the program counter. If location is in
a different function from the one currently executing, the results may
be bizarre if the two functions expect different patterns of arguments or
of local variables. For this reason, the jump command requests
confirmation if the specified line is not in the function currently
executing. However, even bizarre results are predictable if you are
well acquainted with the machine-language code of your program.

On many systems, you can get much the same effect as the jump
command by storing a new value into the register $pc. The
difference is that this does not start your program running; it only
changes the address of where it will run when you continue. For
example,

set $pc = 0x485

makes the next continue command or stepping command execute at
address 0x485, rather than at the address where your program stopped.
See Continuing and Stepping.

The most common occasion to use the jump command is to back
up—perhaps with more breakpoints set—over a portion of a program
that has already executed, in order to examine its execution in more
detail.

17.3 Giving your Program a Signal

signal signal

Resume execution where your program is stopped, but immediately give it the
signal signal. The signal can be the name or the number of a
signal. For example, on many systems signal 2 and signal
SIGINT are both ways of sending an interrupt signal.

Alternatively, if signal is zero, continue execution without
giving a signal. This is useful when your program stopped on account of
a signal and would ordinarily see the signal when resumed with the
continue command; ‘signal 0’ causes it to resume without a
signal.

Note: When resuming a multi-threaded program, signal is
delivered to the currently selected thread, not the thread that last
reported a stop. This includes the situation where a thread was
stopped due to a signal. So if you want to continue execution
suppressing the signal that stopped a thread, you should select that
same thread before issuing the ‘signal 0’ command. If you issue
the ‘signal 0’ command with another thread as the selected one,
GDB detects that and asks for confirmation.

Invoking the signal command is not the same as invoking the
kill utility from the shell. Sending a signal with kill
causes GDB to decide what to do with the signal depending on
the signal handling tables (see Signals). The signal command
passes the signal directly to your program.

signal does not repeat when you press RET a second time
after executing the command.

queue-signal signal

Queue signal to be delivered immediately to the current thread
when execution of the thread resumes. The signal can be the name or
the number of a signal. For example, on many systems signal 2 and
signal SIGINT are both ways of sending an interrupt signal.
The handling of the signal must be set to pass the signal to the program,
otherwise GDB will report an error.
You can control the handling of signals from GDB with the
handle command (see Signals).

Alternatively, if signal is zero, any currently queued signal
for the current thread is discarded and when execution resumes no signal
will be delivered. This is useful when your program stopped on account
of a signal and would ordinarily see the signal when resumed with the
continue command.

This command differs from the signal command in that the signal
is just queued, execution is not resumed. And queue-signal cannot
be used to pass a signal whose handling state has been set to nopass
(see Signals).

17.4 Returning from a Function

return

return expression

You can cancel execution of a function call with the return
command. If you give an
expression argument, its value is used as the function’s return
value.

When you use return, GDB discards the selected stack frame
(and all frames within it). You can think of this as making the
discarded frame return prematurely. If you wish to specify a value to
be returned, give that value as the argument to return.

This pops the selected stack frame (see Selecting a
Frame), and any other frames inside of it, leaving its caller as the
innermost remaining frame. That frame becomes selected. The
specified value is stored in the registers used for returning values
of functions.

The return command does not resume execution; it leaves the
program stopped in the state that would exist if the function had just
returned. In contrast, the finish command (see Continuing and Stepping) resumes execution until the
selected stack frame returns naturally.

GDB needs to know how the expression argument should be set for
the inferior. The concrete registers assignment depends on the OS ABI and the
type being returned by the selected stack frame. For example it is common for
OS ABI to return floating point values in FPU registers while integer values in
CPU registers. Still some ABIs return even floating point values in CPU
registers. Larger integer widths (such as long long int) also have
specific placement rules. GDB already knows the OS ABI from its
current target so it needs to find out also the type being returned to make the
assignment into the right register(s).

Normally, the selected stack frame has debug info. GDB will always
use the debug info instead of the implicit type of expression when the
debug info is available. For example, if you type return -1, and the
function in the current stack frame is declared to return a long long
int, GDB transparently converts the implicit int value of -1
into a long long int:

However, if the selected stack frame does not have a debug info, e.g., if the
function was compiled without debug info, GDB has to find out the type
to return from user. Specifying a different type by mistake may set the value
in different inferior registers than the caller code expects. For example,
typing return -1 with its implicit type int would set only a part
of a long long int result for a debug info less function (on 32-bit
architectures). Therefore the user is required to specify the return type by
an appropriate cast explicitly:

17.5 Calling Program Functions

Evaluate the expression expr and display the resulting value.
The expression may include calls to functions in the program being
debugged.

call expr

Evaluate the expression expr without displaying void
returned values.

You can use this variant of the print command if you want to
execute a function from your program that does not return anything
(a.k.a. a void function), but without cluttering the output
with void returned values that GDB will otherwise
print. If the result is not void, it is printed and saved in the
value history.

It is possible for the function you call via the print or
call command to generate a signal (e.g., if there’s a bug in
the function, or if you passed it incorrect arguments). What happens
in that case is controlled by the set unwindonsignal command.

Similarly, with a C++ program it is possible for the function you
call via the print or call command to generate an
exception that is not handled due to the constraints of the dummy
frame. In this case, any exception that is raised in the frame, but has
an out-of-frame exception handler will not be found. GDB builds a
dummy-frame for the inferior function call, and the unwinder cannot
seek for exception handlers outside of this dummy-frame. What happens
in that case is controlled by the
set unwind-on-terminating-exception command.

set unwindonsignal

Set unwinding of the stack if a signal is received while in a function
that GDB called in the program being debugged. If set to on,
GDB unwinds the stack it created for the call and restores
the context to what it was before the call. If set to off (the
default), GDB stops in the frame where the signal was
received.

show unwindonsignal

Show the current setting of stack unwinding in the functions called by
GDB.

set unwind-on-terminating-exception

Set unwinding of the stack if a C++ exception is raised, but left
unhandled while in a function that GDB called in the program being
debugged. If set to on (the default), GDB unwinds the stack
it created for the call and restores the context to what it was before
the call. If set to off, GDB the exception is delivered to
the default C++ exception handler and the inferior terminated.

show unwind-on-terminating-exception

Show the current setting of stack unwinding in the functions called by
GDB.

17.5.1 Calling functions with no debug info

Sometimes, a function you wish to call is missing debug information.
In such case, GDB does not know the type of the function,
including the types of the function’s parameters. To avoid calling
the inferior function incorrectly, which could result in the called
function functioning erroneously and even crash, GDB refuses
to call the function unless you tell it the type of the function.

For prototyped (i.e. ANSI/ISO style) functions, there are two ways
to do that. The simplest is to cast the call to the function’s
declared return type. For example:

Casting the return type of a no-debug function is equivalent to
casting the function to a pointer to a prototyped function that has a
prototype that matches the types of the passed-in arguments, and
calling that. I.e., the call above is equivalent to:

If the function you wish to call is declared as unprototyped (i.e.
old K&R style), you must use the cast-to-function-pointer syntax, so
that GDB knows that it needs to apply default argument
promotions (promote float arguments to double). See float
promotion. For example, given this unprototyped C function with
float parameters, and no debug info:

17.6 Patching Programs

By default, GDB opens the file containing your program’s
executable code (or the corefile) read-only. This prevents accidental
alterations to machine code; but it also prevents you from intentionally
patching your program’s binary.

If you’d like to be able to patch the binary, you can specify that
explicitly with the set write command. For example, you might
want to turn on internal debugging flags, or even to make emergency
repairs.

set write on

set write off

If you specify ‘set write on’, GDB opens executable and
core files for both reading and writing; if you specify set write
off (the default), GDB opens them read-only.

If you have already loaded a file, you must load it again (using the
exec-file or core-file command) after changing set
write, for your new setting to take effect.

show write

Display whether executable files and core files are opened for writing
as well as reading.

17.7 Compiling and injecting code in GDB

GDB supports on-demand compilation and code injection into
programs running under GDB. GCC 5.0 or higher built with
libcc1.so must be installed for this functionality to be enabled.
This functionality is implemented with the following commands.

compile code source-code

compile code -raw --source-code

Compile source-code with the compiler language found as the current
language in GDB (see Languages). If compilation and
injection is not supported with the current language specified in
GDB, or the compiler does not support this feature, an error
message will be printed. If source-code compiles and links
successfully, GDB will load the object-code emitted,
and execute it within the context of the currently selected inferior.
It is important to note that the compiled code is executed immediately.
After execution, the compiled code is removed from GDB and any
new types or variables you have defined will be deleted.

The command allows you to specify source-code in two ways.
The simplest method is to provide a single line of code to the command.
E.g.:

compile code printf ("hello world\n");

If you specify options on the command line as well as source code, they
may conflict. The ‘--’ delimiter can be used to separate options
from actual source code. E.g.:

compile code -r -- printf ("hello world\n");

Alternatively you can enter source code as multiple lines of text. To
enter this mode, invoke the ‘compile code’ command without any text
following the command. This will start the multiple-line editor and
allow you to type as many lines of source code as required. When you
have completed typing, enter ‘end’ on its own line to exit the
editor.

compile code
>printf ("hello\n");
>printf ("world\n");
>end

Specifying ‘-raw’, prohibits GDB from wrapping the
provided source-code in a callable scope. In this case, you must
specify the entry point of the code by defining a function named
_gdb_expr_. The ‘-raw’ code cannot access variables of the
inferior. Using ‘-raw’ option may be needed for example when
source-code requires ‘#include’ lines which may conflict with
inferior symbols otherwise.

compile file filename

compile file -raw filename

Like compile code, but take the source code from filename.

compile file /home/user/example.c

compile print expr

compile print /fexpr

Compile and execute expr with the compiler language found as the
current language in GDB (see Languages). By default the
value of expr is printed in a format appropriate to its data type;
you can choose a different format by specifying ‘/f’, where
f is a letter specifying the format; see Output
Formats.

compile print

compile print /f

Alternatively you can enter the expression (source code producing it) as
multiple lines of text. To enter this mode, invoke the ‘compile print’
command without any text following the command. This will start the
multiple-line editor.

The process of compiling and injecting the code can be inspected using:

set debug compile

Turns on or off display of GDB process of compiling and
injecting the code. The default is off.

show debug compile

Displays the current state of displaying GDB process of
compiling and injecting the code.

17.7.1 Compilation options for the compile command

GDB needs to specify the right compilation options for the code
to be injected, in part to make its ABI compatible with the inferior
and in part to make the injected code compatible with GDB’s
injecting process.

The options used, in increasing precedence:

target architecture and OS options (gdbarch)

These options depend on target processor type and target operating
system, usually they specify at least 32-bit (-m32) or 64-bit
(-m64) compilation option.

compilation options recorded in the target

GCC (since version 4.7) stores the options used for compilation
into DW_AT_producer part of DWARF debugging information according
to the GCC option -grecord-gcc-switches. One has to
explicitly specify -g during inferior compilation otherwise
GCC produces no DWARF. This feature is only relevant for
platforms where -g produces DWARF by default, otherwise one may
try to enforce DWARF by using -gdwarf-4.

compilation options set by set compile-args

You can override compilation options using the following command:

set compile-args

Set compilation options used for compiling and injecting code with the
compile commands. These options override any conflicting ones
from the target architecture and/or options stored during inferior
compilation.

show compile-args

Displays the current state of compilation options override.
This does not show all the options actually used during compilation,
use set debug compile for that.

17.7.2 Caveats when using the compile command

There are a few caveats to keep in mind when using the compile
command. As the caveats are different per language, the table below
highlights specific issues on a per language basis.

C code examples and caveats

When the language in GDB is set to ‘C’, the compiler will
attempt to compile the source code with a ‘C’ compiler. The source
code provided to the compile command will have much the same
access to variables and types as it normally would if it were part of
the program currently being debugged in GDB.

Below is a sample program that forms the basis of the examples that
follow. This program has been compiled and loaded into GDB,
much like any other normal debugging session.

For the purposes of the examples in this section, the program above has
been compiled, loaded into GDB, stopped at the function
main, and GDB is awaiting input from the user.

To access variables and types for any program in GDB, the
program must be compiled and packaged with debug information. The
compile command is not an exception to this rule. Without debug
information, you can still use the compile command, but you will
be very limited in what variables and types you can access.

So with that in mind, the example above has been compiled with debug
information enabled. The compile command will have access to
all variables and types (except those that may have been optimized
out). Currently, as GDB has stopped the program in the
main function, the compile command would have access to
the variable k. You could invoke the compile command
and type some source code to set the value of k. You can also
read it, or do anything with that variable you would normally do in
C. Be aware that changes to inferior variables in the
compile command are persistent. In the following example:

compile code k = 3;

the variable k is now 3. It will retain that value until
something else in the example program changes it, or another
compile command changes it.

Normal scope and access rules apply to source code compiled and
injected by the compile command. In the example, the variables
j and k are not accessible yet, because the program is
currently stopped in the main function, where these variables
are not in scope. Therefore, the following command

compile code j = 3;

will result in a compilation error message.

Once the program is continued, execution will bring these variables in
scope, and they will become accessible; then the code you specify via
the compile command will be able to access them.

You can create variables and types with the compile command as
part of your source code. Variables and types that are created as part
of the compile command are not visible to the rest of the program for
the duration of its run. This example is valid:

compile code int ff = 5; printf ("ff is %d\n", ff);

However, if you were to type the following into GDB after that
command has completed:

compile code printf ("ff is %d\n'', ff);

a compiler error would be raised as the variable ff no longer
exists. Object code generated and injected by the compile
command is removed when its execution ends. Caution is advised
when assigning to program variables values of variables created by the
code submitted to the compile command. This example is valid:

compile code int ff = 5; k = ff;

The value of the variable ff is assigned to k. The variable
k does not require the existence of ff to maintain the value
it has been assigned. However, pointers require particular care in
assignment. If the source code compiled with the compile command
changed the address of a pointer in the example program, perhaps to a
variable created in the compile command, that pointer would point
to an invalid location when the command exits. The following example
would likely cause issues with your debugged program:

compile code int ff = 5; p = &ff;

In this example, p would point to ff when the
compile command is executing the source code provided to it.
However, as variables in the (example) program persist with their
assigned values, the variable p would point to an invalid
location when the command exists. A general rule should be followed
in that you should either assign NULL to any assigned pointers,
or restore a valid location to the pointer before the command exits.

Similar caution must be exercised with any structs, unions, and typedefs
defined in compile command. Types defined in the compile
command will no longer be available in the next compile command.
Therefore, if you cast a variable to a type defined in the
compile command, care must be taken to ensure that any future
need to resolve the type can be achieved.

Variables that have been optimized away by the compiler are not
accessible to the code submitted to the compile command.
Access to those variables will generate a compiler error which GDB
will print to the console.

17.7.3 Compiler search for the compile command

GDB needs to find GCC for the inferior being debugged
which may not be obvious for remote targets of different architecture
than where GDB is running. Environment variable PATH on
GDB host is searched for GCC binary matching the
target architecture and operating system. This search can be overriden
by set compile-gccGDB command below. PATH is
taken from shell that executed GDB, it is not the value set by
GDB command set environment). See Environment.

Specifically PATH is searched for binaries matching regular expression
arch(-[^-]*)?-os-gcc according to the inferior target being
debugged. arch is processor name — multiarch is supported, so for
example both i386 and x86_64 targets look for pattern
(x86_64|i.86) and both s390 and s390x targets look
for pattern s390x?. os is currently supported only for
pattern linux(-gnu)?.

On Posix hosts the compiler driver GDB needs to find also
shared library libcc1.so from the compiler. It is searched in
default shared library search path (overridable with usual environment
variable LD_LIBRARY_PATH), unrelated to PATH or set
compile-gcc settings. Contrary to it libcc1plugin.so is found
according to the installation of the found compiler — as possibly
specified by the set compile-gcc command.

set compile-gcc

Set compilation command used for compiling and injecting code with the
compile commands. If this option is not set (it is set to
an empty string), the search described above will occur — that is the
default.

show compile-gcc

Displays the current compile command GCC driver filename.
If set, it is the main command gcc, found usually for example
under name x86_64-linux-gnu-gcc.

18 GDB Files

GDB needs to know the file name of the program to be debugged,
both in order to read its symbol table and in order to start your
program. To debug a core dump of a previous run, you must also tell
GDB the name of the core dump file.

18.1 Commands to Specify Files

You may want to specify executable and core dump file names. The usual
way to do this is at start-up time, using the arguments to
GDB’s start-up commands (see Getting In and
Out of GDB).

Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to
specify a file you want to use. Or you are debugging a remote target
via gdbserver (see Using the gdbserver
Program). In these situations the GDB commands to specify
new files are useful.

file filename

Use filename as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the program
executed when you use the run command. If you do not specify a
directory and the file is not found in the GDB working directory,
GDB uses the environment variable PATH as a list of
directories to search, just as the shell does when looking for a program
to run. You can change the value of this variable, for both GDB
and your program, using the path command.

You can load unlinked object .o files into GDB using
the file command. You will not be able to “run” an object
file, but you can disassemble functions and inspect variables. Also,
if the underlying BFD functionality supports it, you could use
gdb -write to patch object files using this technique. Note
that GDB can neither interpret nor modify relocations in this
case, so branches and some initialized variables will appear to go to
the wrong place. But this feature is still handy from time to time.

file

file with no argument makes GDB discard any information it
has on both executable file and the symbol table.

exec-file [filename]

Specify that the program to be run (but not the symbol table) is found
in filename. GDB searches the environment variable PATH
if necessary to locate your program. Omitting filename means to
discard information on the executable file.

symbol-file [filename[ -o offset]]

Read symbol table information from file filename. PATH is
searched when necessary. Use the file command to get both symbol
table and program to run from the same file.

If an optional offset is specified, it is added to the start
address of each section in the symbol file. This is useful if the
program is relocated at runtime, such as the Linux kernel with kASLR
enabled.

symbol-file with no argument clears out GDB information on your
program’s symbol table.

The symbol-file command causes GDB to forget the contents of
some breakpoints and auto-display expressions. This is because they may
contain pointers to the internal data recording symbols and data types,
which are part of the old symbol table data being discarded inside
GDB.

symbol-file does not repeat if you press RET again after
executing it once.

When GDB is configured for a particular environment, it
understands debugging information in whatever format is the standard
generated for that environment; you may use either a GNU compiler, or
other compilers that adhere to the local conventions.
Best results are usually obtained from GNU compilers; for example,
using GCC you can generate debugging information for
optimized code.

For most kinds of object files, with the exception of old SVR3 systems
using COFF, the symbol-file command does not normally read the
symbol table in full right away. Instead, it scans the symbol table
quickly to find which source files and which symbols are present. The
details are read later, one source file at a time, as they are needed.

The purpose of this two-stage reading strategy is to make GDB
start up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular source
file are being read. (The set verbose command can turn these
pauses into messages if desired. See Optional
Warnings and Messages.)

We have not implemented the two-stage strategy for COFF yet. When the
symbol table is stored in COFF format, symbol-file reads the
symbol table data in full right away. Note that “stabs-in-COFF”
still does the two-stage strategy, since the debug info is actually
in stabs format.

symbol-file [ -readnow ]filename

file [ -readnow ]filename

You can override the GDB two-stage strategy for reading symbol
tables by using the ‘-readnow’ option with any of the commands that
load symbol table information, if you want to be sure GDB has the
entire symbol table available.

symbol-file [ -readnever ]filename

file [ -readnever ]filename

You can instruct GDB to never read the symbolic information
contained in filename by using the ‘-readnever’ option.
See --readnever.

core-file [filename]

core

Specify the whereabouts of a core dump file to be used as the “contents
of memory”. Traditionally, core files contain only some parts of the
address space of the process that generated them; GDB can access the
executable file itself for other parts.

core-file with no argument specifies that no core file is
to be used.

Note that the core file is ignored when your program is actually running
under GDB. So, if you have been running your program and you
wish to debug a core file instead, you must kill the subprocess in which
the program is running. To do this, use the kill command
(see Killing the Child Process).

The add-symbol-file command reads additional symbol table
information from the file filename. You would use this command
when filename has been dynamically loaded (by some other means)
into the program that is running. The textaddress parameter gives
the memory address at which the file’s text section has been loaded.
You can additionally specify the base address of other sections using
an arbitrary number of ‘-s sectionaddress’ pairs.
If a section is omitted, GDB will use its default addresses
as found in filename. Any address or textaddress
can be given as an expression.

If an optional offset is specified, it is added to the start
address of each section, except those for which the address was
specified explicitly.

The symbol table of the file filename is added to the symbol table
originally read with the symbol-file command. You can use the
add-symbol-file command any number of times; the new symbol data
thus read is kept in addition to the old.

Changes can be reverted using the command remove-symbol-file.

Although filename is typically a shared library file, an
executable file, or some other object file which has been fully
relocated for loading into a process, you can also load symbolic
information from relocatable .o files, as long as:

the file’s symbolic information refers only to linker symbols defined in
that file, not to symbols defined by other object files,

every section the file’s symbolic information refers to has actually
been loaded into the inferior, as it appears in the file, and

you can determine the address at which every section was loaded, and
provide these to the add-symbol-file command.

Some embedded operating systems, like Sun Chorus and VxWorks, can load
relocatable files into an already running program; such systems
typically make the requirements above easy to meet. However, it’s
important to recognize that many native systems use complex link
procedures (.linkonce section factoring and C++ constructor table
assembly, for example) that make the requirements difficult to meet. In
general, one cannot assume that using add-symbol-file to read a
relocatable object file’s symbolic information will have the same effect
as linking the relocatable object file into the program in the normal
way.

add-symbol-file does not repeat if you press RET after using it.

remove-symbol-file filename

remove-symbol-file -a address

Remove a symbol file added via the add-symbol-file command. The
file to remove can be identified by its filename or by an address
that lies within the boundaries of this symbol file in memory. Example:

Load symbols from the given address in a dynamically loaded
object file whose image is mapped directly into the inferior’s memory.
For example, the Linux kernel maps a syscall DSO into each
process’s address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file’s shared object file header.
For this command to work, you must have used symbol-file or
exec-file commands in advance.

section sectionaddr

The section command changes the base address of the named
section of the exec file to addr. This can be used if the
exec file does not contain section addresses, (such as in the
a.out format), or when the addresses specified in the file
itself are wrong. Each section must be changed separately. The
info files command, described below, lists all the sections and
their addresses.

info files

info target

info files and info target are synonymous; both print the
current target (see Specifying a Debugging Target),
including the names of the executable and core dump files currently in
use by GDB, and the files from which symbols were loaded. The
command help target lists all possible targets rather than
current ones.

maint info sections

Another command that can give you extra information about program sections
is maint info sections. In addition to the section information
displayed by info files, this command displays the flags and file
offset of each section in the executable and core dump files. In addition,
maint info sections provides the following command options (which
may be arbitrarily combined):

ALLOBJ

Display sections for all loaded object files, including shared libraries.

sections

Display info only for named sections.

section-flags

Display info only for sections for which section-flags are true.
The section flags that GDB currently knows about are:

ALLOC

Section will have space allocated in the process when loaded.
Set for all sections except those containing debug information.

LOAD

Section will be loaded from the file into the child process memory.
Set for pre-initialized code and data, clear for .bss sections.

RELOC

Section needs to be relocated before loading.

READONLY

Section cannot be modified by the child process.

CODE

Section contains executable code only.

DATA

Section contains data only (no executable code).

ROM

Section will reside in ROM.

CONSTRUCTOR

Section contains data for constructor/destructor lists.

HAS_CONTENTS

Section is not empty.

NEVER_LOAD

An instruction to the linker to not output the section.

COFF_SHARED_LIBRARY

A notification to the linker that the section contains
COFF shared library information.

IS_COMMON

Section contains common symbols.

set trust-readonly-sections on

Tell GDB that readonly sections in your object file
really are read-only (i.e. that their contents will not change).
In that case, GDB can fetch values from these sections
out of the object file, rather than from the target program.
For some targets (notably embedded ones), this can be a significant
enhancement to debugging performance.

The default is off.

set trust-readonly-sections off

Tell GDB not to trust readonly sections. This means that
the contents of the section might change while the program is running,
and must therefore be fetched from the target when needed.

show trust-readonly-sections

Show the current setting of trusting readonly sections.

All file-specifying commands allow both absolute and relative file names
as arguments. GDB always converts the file name to an absolute file
name and remembers it that way.

On MS-Windows GDB must be linked with the Expat library to support
shared libraries. See Expat.

GDB automatically loads symbol definitions from shared libraries
when you use the run command, or when you examine a core file.
(Before you issue the run command, GDB does not understand
references to a function in a shared library, however—unless you are
debugging a core file).

There are times, however, when you may wish to not automatically load
symbol definitions from shared libraries, such as when they are
particularly large or there are many of them.

To control the automatic loading of shared library symbols, use the
commands:

set auto-solib-add mode

If mode is on, symbols from all shared object libraries
will be loaded automatically when the inferior begins execution, you
attach to an independently started inferior, or when the dynamic linker
informs GDB that a new library has been loaded. If mode
is off, symbols must be loaded manually, using the
sharedlibrary command. The default value is on.

If your program uses lots of shared libraries with debug info that
takes large amounts of memory, you can decrease the GDB
memory footprint by preventing it from automatically loading the
symbols from shared libraries. To that end, type set
auto-solib-add off before running t